@ ehmen,
It is quite possible to update to 1709 without disabling Avast.
I disabled it only because it makes the likelihood of a problem less likely.
It also speeds up the process. I actually disabled all programs that do any kind of scanning
before I started the process.
This is all personal preference. You need to be your own judge on this.
Since you don't seem to have any problems, why are you concerned
To phrase it differently,
"If you have no problems, why are you looking for one"