There's a good article about twist rates and accuracy in one of my Varmint magazines. Very interesting. I'll try to summarize their major points. A slow twist, as we all know, will stabilize small (short) bullets. That would be 1 in 12 or 1 in 14. The 1 in 9 was considered to be the best 'general' twist for 35 to 70ish gr bullets. I don't remember what they said about 1 in 8, but they did say that going to 1 in 7 or 1 in 6.5 was mainly for the folks shooting long range with specialty bullets of up to 90 grains. So, if shooting a range of bullets from 35 to 70ish grains, you are good with a 1 in 9 twist. If you need to shoot somewhat heavier bullets, go with the 1 in 8 twist. Seems like most rifle makers go with a 9 twist or an 8 twist, so it appears that they are well aware of what works with those twist rates.
The above is just a rehash of what I read with great interest. There are probably folks among us that can speak from actual experience and greater detail.
My new 223 barrel, which I hope to have in my hands soon, will be a 1 in 9 twist. If I had any intention to shoot bullets in the 75 to 77 gr range, I would have gone with a 1 in 8.