I'll usually twist my heel outwards to release my SPDs, but it's possible to release by twisting my heel in too - although I don't know if that's only because I use the multi-release cleats. I tend to only release inwards if, say, I've dismounted the bike to one side and decided to get off completely rather than move away - it's sometimes easier to be able to twist your heel inwards with one leg on the ground and the other over the crossbar. It also helps if you've fallen off, lol