Every 2- 3,000 miles is good. If you're changing the oil every 2-3,000 miles then do them together. And if you're doing the work yourself I recommend keeping a journal on what valves need adjustment at each service; if you see a pattern of 1 valve moving consistently more than the others you'll know something is wrong and have to make plans to check that head out. The valve stem could be stretching or the seat may be getting pounded into the head- both rare, but they do happen. Exhaust valves especially (because they get sooo hot!) depend on the proper amount of time sitting on the seat for cooling and survival so too tight is not good (the exhaust valve face and even the seat can corrode/burn), but if the adjustment is too loose (exhaust or intake) then the engine doesn't make the power it was designed to.
"Whenever you notice something amiss" is too late- valve adjustments are preventative maintenance, and yes, "if it ain't broke don't fix it" has no place here. A new/rebuilt engine or one that has had headwork done will require several valve adjustments in the first 2,000 or so miles, as parts are wearing into each other. On new engines some guys will adjust valves 3, 4 or even 5 times in the first 500-1,000 miles- after cam break-in (20 minutes running above 2,000 rpm, shut it off and let it cool), after the first 30-50 miles, at 300 and then 1,000 miles. If you're super paranoid throw a couple more in the mix. And if you're super, SUPER paranoid- change the oil every time as well.
Hope this helps. Al
PS- Alan means aluminum or chromoly pushrods