I only noticed that when i made the switch to 7- i'm sure it was happening with my 6's too, but it was probably a combination of being more pronounced from the added mass in the bridge, and sheer dumb luck.
And actually, i personally feel they should be tuned strapped on in playing position, not absolutely vertical. Grab a guitar and look at the way it hangs off you- it's not QUITE vertical, but slants in towards your body slightly. We're probably talking about differences so slight that the ear won't pick them up, but this plus a 98% stable trem might be enough to push it into audible sourness... and if it's an easy fix, better safe than sorry.
I think it's kinda cool, actually... "My guitar's bridge is so sensitive that i have to tune it strapped on, so gravity isn't a factor." Yep, i'm a dork.