These are pretty small tank, so the risks are not nearly as much (much less weight.)
Personally, I wouldn't worry too much about a 1/4" difference in a 7 gal
. <1/4" would be my limit for an uneven tank tho ...> But to be perfectly safe, you can shim up the dresser so it is perfectly level.
A 1/8" bow is not bad for a 20 gal
, esp acrylic ... which can have a bit of a give. I think you would be OK.