Wow, really?
Well, this thread is a little empty, both in heart and content.
Short answer: Yes, 1:7 and 1:8 will be so minutely close that a 1:7 barrel is "good enough", don't give it a second thought. You won't ever notice the difference at a match.
Long answer: OMFG, WHAT WERE YOU THINKING???!!!! 1:7???!!!
Let's assume your talking about .223/5.56, you didn't specify caliber, but it's a million to one odds. Heavy metal rifles in .308 have much slower twist rates than .223 ARs for the most part.
What ammo do you shoot? Cheap bulk factory by the thousand? Then 55gr it is. Other cheap bulk, 62gr steel core is a no-go for 3-gun targets. The only stuff you'll find in heavier weights (75,77) for cheap is ruskie steel case, which will give you more extraction fits, and is generally poo-pooed by us patriotic Americans.
For 55gr, the slower twist rates will be better suited, but only slightly. At the typical distances stages will be set up, you won't even be able to overstabilize varmint grain weight bullets so bad that it matters at all.
Next, how are accurate do you consider yourself? Remember, 3-gun rifle targets are generally engaged in one of 3 positions; offhand, improvised prone, and improvised barrier support. These are not inherently accurate positions and you don't have a lot of time to futz with them, so your technique will affect your accuracy 1000x more than the difference between 1:7 and 1:8.