o rings are nice because you can re-use them, if you take a cover off, put it back on, take it off, put it back on... while you are debugging a problem, it will re-seal every time (a gasket should only be used once).
but when the o ring gets old it becomes stiff, and then it needs to be replaced.
The tappet cover o ring leaks are common - the bike usually does not need the tappets checked for a couple years, so if you dont replace the o rings there is a good chance at least one will ooze or outright drip on the garage floor (all the way down the side of the engine).
The problem is: checking the valves is a 2 hour job if you have done it a few times, have to take the carbs off to get to the intake tappets... so if you re-use the $4 each o rings, and the inner ones leak, its another 2 hours to take the carbs off again.
If you modify or leave off the fake plastic valve covers you can get to both exhaust tappets without taking anything off the bike, at least on a 650 and 1100. Its a short cut, you can check the exhaust tappets and if they are out of spec, then do the intake tappets too.
BTW, the service manual says to take off the head side covers so you can see the timing marks precisely on the cam gears. If you have done them once then you can probably only use the timing marks on the crankshaft, and not have to replace the o rings on the top side covers too.