I looked into this when choosing a new to me integrated tube amp for my vintage 16 ohm speakers.
General rule: use the tap closest to the 'nominal' impedance of the speakers, thus use the 8 ohm taps for your 15 ohm speakers.
It will be fine, a slight shift in frequency response will occur as the speaker's crossover was designed for 16 ohm taps. you may or may not hear the difference, as I said, slight.
Guitarists sometimes use different impedances purposely to get a 'different' 'preferred' sound for the specific song(s) or in the specific space they are playing.
I bought my version of Cayin A88T because it has 16 ohm taps as well as 8 and 4. I could have accepted the slight change, then the newer models with 4 and 8 (no 16) have external bias adjusters and meters.
Ralph (atmasphere) stated that my Cayin will have less distortion at it's 16 ohm taps. I remember that, but not the explanation.