It depends on how close to finish grind you want to be before heat treatment. If you have to grind a great deal after heat treat - it doesn’t really matter what grit you stop at before heat treat. You are going to grind it all off anyways.
Depends on the blade dimensions, steel chose and purpose.
I dont want to crack or warp a thin blade ground down to finish or near finish but I also don't want a big heavy blade with a massive amount of grinding yet to be done, especially if forged from a highly wear resistant steel.
Depends on thickness of the steel used and on forged or material removal. Thin material removal knives, nothing on the flats, profile with 220 (this is to prevent warping), thicker material removal knives, 400 plain carbon blades 600 on Damascus. Forged blades I try never to get thinner than a dime on the cutting edge, grind up to 220 grit then heat treat, and do my finish grind routine, 220G, A100, A65, A45 again depending on purpose of knife up to 800ish max (A35 or A30). Then hand sand with last belt grit parallel to blade to get rid of belt grind lines. I don't like mirror grinds on my knives. I want them to be used as intended and the mirrored finish as good as it looks tends to make most people not use them because they don't want to scratch them (please note this is all my opinion!). This is just what works for me.
I have found that anything under 120X runs the risk of stress riser fracturing, if the blade reaches maximum hardness. I teach most of my students to take them to 220X. Less than this will be tougher to remove when the blade is hard, and I discourage those not very used to things like wet grinding to do too much heavy grinding after heat treatment. On the other hand, if they go finer than 220X they will probably just end up back at 220X to remove scale, decarb, and other effects of the heat treatment unless they really have atmosphere under control.
I, personally, will go to 400X or even 600X before heat treatment, due to the equipment I use to control atmospheric effects. I have simply done hand finishing after heat treatment in the past.
If heat treating via methods that most folks use (forge/torch/heat treat oven for heating) I'm with Kevin! At LEAST 120 grit finish....for the reasons he stated. Again, like Kevin, in some instances, I will go to final finish grit before heat treating when I am using the salt tank....blades come out so "clean", that only some light hand finishing is necessary to finish. I often do this with folder blades, since tolerances have to be much tighter than straight blades.