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Depends on the steel you're using and exactly how you're heat treating it. Steels like 1095 are 'sgallow hardening' steels, and have to drop in temperature extremely fast in order to get as hard as they should, and the temperature drops faster the thinner the cross section is. If you're quenching in something that's right at the edge, like warn canola oil, grinding the edge down to about the thickness of a dime will give you the best chances of reaching full hardness.
Deeper hardening steels like O1 and most stainless steel are formulated to reach full hardness with slower quenches, think 2-3 seconds to drop from 1500f to 900f instead of 1 (not exact numbers). Steels like that, you don't even need to grind in the bevels before hand, you're pretty well certain to harden the blade throughout.
Personally, I don't bevel before heat treatment for any steel I use, mostly 1095 and AEB-L stainless. For the 1095 I actually use an engineered quench called Parks 50. Expensive stuff, but it cools the steel nearly as fast as a straight water quench with less risk. Fast as it cools, getting 1/8" thick stuff to harden completely is a non-issue. AEB-L is technically an air hardening steel, so there again, 1/8" thick hardenes with no real effort. I hate grinding bevels, so this way I only have to do it once.
If you're just starting out though, and working with plain carbon steels like 1095, 1075, that stuff, I'd take the edges to about the thickness of a dime before heat treat. Get used to the steel, how it responds to heat treat, then decide if you want to bevel before heat treat or not. No substitute for experience when it comes to HT after all
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