ORIGINAL: da Rock
You don't have to tighten the spinner down overly tight at all. Matter of fact, tightening the spinner cone has little or no effect on how tightly the backplate and prop are held to the thrust washer. So even if you are scared of your 4cycles backfiring everything loose, don't panic and overtighten the spinner cone hold-down screws.
Eh, I'm cognizant that the spinner cone has no effect on the prop washer.
I've had the spinner itself come flying off and hit me in the head, which caused me to tighten it down firmly to prevent a repeat... and ultimately resulted in it cracking.
Maybe a little loc-tite would have been a better solution.