I ran a scenario back in January (under the 4.0 Guide) where the players opted to play up to rather disastrous results. In the wake of that session I was told the group didn't qualify to play up. I've spoken to several of the players, and they all believe it was legal for them to play up.
The party was three 2nd levels PCs and three 1st level PCs. Our APL calculation was thus:
9 total levels divided by 6 PCs = 1.5
+1 to APL for 6 or more PCs = 2.5
The rounding is in question. We rounded 2.5 up to 3, thus allowing us to play up to subtier 4-5.
I did ask about APL before I ran the game. The Venture Captain who responded did specifically say to round decimals of .5 or lower down, and decimals of .6 or higher up. The VC's information is being questioned by at least one player, however, because there isn't any exact text saying that in either version 4.0 or 4.1 of the Guide.
The 4.0 Guide did say in one place to always round APL down. The example though, said to round up. That contradiction is why I asked. The 4.1 Guide simply says to "round to the nearest whole number". As mentioned previously, .5 is exactly between two whole numbers.
(To avoid any confusion, I did not purposely ignore the VC's information. In the stressful rush of prepping to run PFS for the first time, I completely misread it. Because we were later than agreed getting started, I also let the players calculate the APL without double checking it myself.)