My thoughts are get the biggest bags you can fill with growing media. If you get one large enough to carry the seedlings thru the summer in a fabric bag, you can plant in the fall. That would eliminate any heat stress on a seedling planted in May during July and August.
How many seedlings do you have? Bigger bags mean you have to have the growing media to get them filled.
If you plant them in May at their final destination, I would add moisture mizer (water absorbing crystals) to the hole. It sure makes a difference during an extended dry spell during hot days.
I don't leave a bag on a seedling when I plant. I take scissors and cut them off. If you don't want to remove the bag, take a box cutter and make the bottom holy and cut plenty of long slits down the sides.
Wayne