I use System 7.6.1 on my Quadras. There are some tricks you can use with 7.6 to make it run some Mac OS 8 apps, so it is debatable whether 8 is actually necessary on these machines. The additional overhead of 8.1 over 7.6.1 isn't tremendously more, though, so it doesn't run too badly on a fast Quadra either if you should decide to use it.

Here's some benchmarks to prove my point.

http://main.system7today.com/articles/til/benchmarks.html

As you can see 8.1 doesn't do badly at all compared to 7.6.1 and blows away later versions, which you wouldn't be able to use anyway since they're PPC only.