If you are wiring just a speaker, in other words look in the speaker cabinet and all you find is a bare speaker with no other components or multiple speakers. The plus and minus terminals will make little difference, one provision is both speakers in a stereo system should be wired the same or the may sound different.

If there is any sort of a cross-over network or amplifier or there are more than one speaker in the cabinet it will make a difference.

If you have a matching pair of speakers and one does not have the plus/minus marking, there is a very good chance the two are identical.