In that respect, Leinster and Connacht will be busting the customary collective gut to stay where they are and move closer to securing an extra home fixture the weekend before the final. The fact that each will be ploughing headlong into an opponent with top-four ambitions of their own makes Round 20 all the more intriguing.
Edinburgh appear at the RDS on Friday evening knowing they have no further margin for error if they are to defy the odds and revive flickering hopes that they can somehow finish their season on familiar territory by going the distance to hosting their very own Final.
Rearranging the landscape above them will take some doing, not least because Leinster have not lost at home in the Guinness PRO12 since the Dragons won there fully 14 months ago.
The mightiest collisions in terms of impact on the top four take place on Saturday - Connacht against Munster in Galway and Scarlets against the Warriors at Llanelli. In other words second against fifth, fourth against third - crucial matches at any time of the season, doubly so with only three rounds to go.
Munster know all about the Connacht revival. Two recent results against their neighbours stand out for their bruised look, the first at the Sports Ground on New Year's Day 2015 when Kieran Marmion, Craig Ronaldson and Aly Muldowney got the tries in a 24-16 win.
Despite the setback, Munster recovered to reach the Guinness PRO12 Final at the end of last season only for the Warriors to spoil Paul O'Connell's farewell to his native province. There is unlikely to be another such recovery in the event of Connacht completing a rare Munster double.
John Muldoon's squad have passed so many major landmarks already this season that another would hardly be that much of a surprise. They know that Munster have not forgotten the sheer audacity of their victory at a packed Thomond Park last November, clinched by Bundee Aki's corner flag acrobatics.
He and Connacht have swept almost all before them in Galway this season. Of twelve matches there against opponents from England, France, Russia, Italy, Scotland, Wales and Ireland, they have won eleven, losing only once - to Ulster on Boxing Day.
After their team's heroics during last weekend's European Challenge Cup quarter-final in Grenoble, the fans will give them a thunderous welcome home. For any team to gain in stature from a setback in Europe requires a special performance.
Connacht's turned out to be so special that they outscored their Top14 opponents 4-3 on tries. Fionn Carr's dancing diagonal run in the last minute almost made it 5-3 in a magnificent final attempt to keep the Irish flag flying in Europe.
They missed it by the narrowest possible margin, one point in 65 and rarely can there have been a crueler drop goal than the one Jonathan Wisniewski nailed five minutes from time. That it happened to be surely the best contest witnessed in Europe all season is another indication of how far Connacht have come.
The same can be said, in a strictly domestic sense, of the Warriors in terms of making the steep climb from the lower half of the table into the top three. They made their most of their stay in Italy by following the previous week's five-pointer against Benetton Treviso with five more from Zebre in Parma.
A seventh straight win, festooned with seven tries distributed among six players, has put the champions within striking distance of Leinster and Connacht, a mere two points behind the joint leaders.
Now they face as severe a test on the road as they will have had all season, in Llanelli. The Scarlets, unable to cope with the resurgent Blues last time out, dare not afford a second successive home setback if they are to make the play-offs for only the second time in seven attempts.
Ulster will expect to capitalise on the duel between the two teams occupying the other play-off positions directly above them by making the most of their visit to Zebre.
The Blues, on a roll under head coach Danny Wilson and making a late run for a possible top-six finish, complete the weekend in Cardiff on Sunday with a local derby against the only Guinness PRO12 team left standing in Europe, the Dragons.
In striking a wonderful blow for the competition, the Dragons came from behind to knock the Challenge Cup holders Gloucester out at Kingsholm. Theirs was a victory for bravery, one clinched by a late try from substitute scrum half Charlie Davies after skipper Lewis Evans had spurned the soft option of an equalising penalty which may, or may not have taken the tie into extra time.
Their reward is a second successive semi-final. After Edinburgh at BT Murrayfield twelve months ago, the Dragons are now confronted by a possibly more daunting trip, to Montpellier but then they said the same about Gloucester and look what happened….
Current form in the Guinness PRO12 (figures for PRO12 matches only):
Winning runs:
7 - Glasgow Warriors.
3 - Cardiff Blues
Losing runs:
8 - Zebre.
7 - Newport Gwent Dragons
4 - Benetton Treviso.
Since a home defeat:
12 - Leinster (last one v Dragons February 15, 2015)
4 - Zebre (Benetton Treviso, December 27, 2015)
3 - Connacht (last one v Ulster, December 26, 2015)
3 - Cardiff Blues (last one v Leinster, February 20, 2016)
Since an away defeat:
3 -Glasgow Warriors (Ulster, February 12, 2016)
2 - Ulster (Benetton Treviso, January 30, 2016)
Since an away win:
13- Benetton Treviso (Zebre, December 27, 2014)
10- Newport Gwent Dragons (Benetton Treviso, March 28, 2015)
2 - Munster (Benetton Treviso, February 28, 2016)
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