I'd honestly say up to you. I assume like Malwarebytes, having that enabled will slow down the scan time. Does Avast by default quick scan the computer upon startup?
After 8 mins of the computer starting up, Avast scans for any active rootkits which are running in memory. This scan is very quick (a few seconds usually in the background) which you won't notice.
I'd suggest you keep it enabled. You can test to see the difference (with and without the setting enabled) but you really won't notice anything.
If your security setup is good enough and your computer was clean before you installed it, then having it disabled should not be a big issue. If a rootkit got installed on a computer the damage is probably already done. That said, having it enabled won't do any harm if your computer has the resources for it.
If I were you, I would just let it enabled since the well known GMER - Standalone anti rootkit scanner uses Avast's anti rootkit technology. Sounds like good to go with enabled on this one.
Rootkit scan at startup is important to detect rootkits before they are loaded. Planning a scan at boot, Avast takes control of the system and, even before Windows starts, run the scan for any possible rootkits to easily eliminating them.
Malware with rootkit capabilities are not very common nowadays, then you can flag the box sometimes, when you want.
After 8 mins of the computer starting up, Avast scans for any active rootkits which are running in memory. This scan is very quick (a few seconds usually in the background) which you won't notice.
If I were you, I would just let it enabled since the well known GMER - Standalone anti rootkit scanner uses Avast's anti rootkit technology. Sounds like good to go with enabled on this one.