i personally would just have it turn on automatically since turning on is what i would have to do then anyway. i think a user should just be aware what layer they are drawing on
Personally, I would avoid all types of pop-ups as they tend to prevent extensions or scripts from working. Sometimes Rhino simply writes in the prompt what could go wrong or has gone wrong. I think this is the best way to inform and allow some freedom. At least a global flag in the options to warn via popups or command prompt. When you get used to a software you need reminders not show-stoppers.
Regards
@dale i saw this is on your list, is there any chance we could have this in v9? it is a small change (i guess and hope) but it would be such a game changer in fiddling with layers for quick overviews. since there have been numerous requests for this it might be a thankful implementation.