No. The messages must be on the safe side. Being so much afraid of closing applications or rebooting is not a normal case; installing a software might ask for such things from time to time. If sometimes it could happen to be safe not to, is just am exception. I disagree that for a user (especially novice), some messages that bring uncertainty (“may be in come times possible not to …”) is better than clear and robust guidance, even if sometimes it might happen to be avoidable.
The primary goal of installer is not to avoid user interactions and interruptions at all costs. The primary goal is to reliably install application, and leave system in a predictable and safe state. All other things are secondary. If possible, it could try to minimize interaction (yes, that’s good, but secondary!). And being not the primary function of the installed application, the possible inconvenience is much lower priority than possible problems when installer fails its primary goal.