I see no point in running a boot-time scan other than in the circumstances mentioned above. It is more thorough (in the areas that it does scan), scans for PUPs and scans Archives, this will mean the boot-time scan is likely to take much longer.
That is the point of the Quick and Full System Scans, they are on default settings. If there are other things that you want to change/include then the creation of a custom scan is what should be done rather than try to change any settings in the default scans.
I see little reason for a custom scan either, I have basically the Quick scan on default settings that is scheduled to run weekly.
- With a resident on-access antivirus like avast, the need for frequent on-demand scans is much depreciated. For the most part the on-demand scan is going to be scanning files that would be otherwise be dormant or inert. If they were active files then the on-access file system shield would be scanning them before being created, modified, opened or executed.
I have avast set to do a scheduled weekly Quick scan, set at a time and day that I know the computer will be on. If for some reason my system wasn't on, no big deal I will catch up on the next scheduled scan.