As a saxophonist/clarinetist turned into a singer, I do have a note along the lines of what PJ mentions: don't hurt yourself.
I've mentioned before that I have a relatively wide vocal range. However, if I'm really a bass (I am) and I decide to sing a lot of tenor I (i.e. "really high") and I'm doing it wrong, I'm going to start damaging my vocal chords. At the very least, this will take some steps off your range and, at the most, you'll end up sounding like Bea Arthur. (Which is good for some types of music, I suppose ....)
When I first started singing, as a church musician, I noticed that it was difficult to get through four or five services on a weekend (i.e. about 2.5 hours of singing) because my throat couldn't handle it: I would either have a 1/2 octave range or just couldn't sing, period. When I started taking lessons and finding out how to hit notes correctly, I had no problems at all singing 2.5 hours. Well, unless I was singing tenor I. Hey, I only did that when a majority of the tenor Is weren't there. That wasn't more than about 4 weekends a month.