Check the chain tension at a number of points in the chain..... if it's within spec, leave it alone. Looser is better than too tight. At the same time, if you can actually hear it slap, rather than just thinking it might, I'd adjust it. Even then, I would keep the tension as near the 'loose' end of the adjustment as possible.
I always checked my chain on the center stand, with the bike OFF (smiles) in neutral.... that way, it's easy to rotate the rear tire so you could check various 'runs' of the chain. As I am sure you are aware, chains don't wear evenly.... once any link gets some slop in the pin/plate connection, that 'jerks' the next link, which starts to make that one wear.... then both of those, will pull on the next one..... The 'tight spot' are the links that have not been worn.
Most of my chains have worn out between 15 and 25K miles. Usually, they go more than half their life without needing any adjustment at all, just lube. Then, they'll need a slight adjustment, 3 months riding later, another, then the adjustments come a little more regularly.
When they get to the point where you need to adjust the chain monthly, it's time to replace.
