You can always do it - whether it's worth it is up to you.

I always do this on the back end of a 7-night award if I have a redeye flight. To me, it's worth it in that case. If it were a full-revenue stay, I'd probably look for another option, such as the fitness center or even paying the basic entry fee to the spa.

Some hotels will also work with you on a half-day rate at the end of your stay, which might effectively get you an early-evening checkout and your late-day shower.