I decided something was amiss when noticing the payout declined posts still show some value behind them, so I decided to test this theory and as it turns out if you upvote payout declined posts it still drains your voting power! The same amount of voting power loss as if you voted on anything else, but you won't be having any curation rewards from that.
Now, of course, I get the idea of your voting anyway after seeing the declined reward strike because I do it too - you want to show some support like a thumbs up even if the author isn't taking the money you still want that like button. So then my suggestion is to just use a 1% vote for these kinds of posts when you still want to upvote it, because then the drain on your voting power will be very tiny and considering the author opted into declined payout I think they will be ok with that too.
Anyway I hope this helps you save a bit of voting power occasionally. If it's not something you already knew about, or think maybe your followers don't already know this, consider a RESTEEM!
Bonus Fact:
upvoting something that has already paid out (past 7 days) does NOT drain voting power :)
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