I do believe in sticking with one teacher.
For a while ! Then you definitly need to change.
It is a very simple equation, really : if you only take lessons with one teacher, no matter how great he is, the best dancer you'll ever get will be a clone of that particular dancer.
If you change teachers, you get a different perspective. You get different ways of teaching and correcting. You might even get, as Squirrel said, an opposit vision of salsa. But by the time you need to change teachers, you need not someone telling you how to feel about salsa. You already have your own opinion, they can influence it, but not change it alltogether.
Sometimes, you will get stuck on a particular move, and you can't get it right. You'll get corrected by your teacher time and time again, but to no avail. You change teacher, and the first thing he'l say will be, "you should try and do this move like this, in slow mo, just to see how it feels twisting your arm the other way" and bam ! Not stuck anymore.
Just a different aproach of explaining things can unlock doors that had been closed for you for weeks !
Besides, you sometimes go back to the basics with a new teacher, and I found it can nevrer hurt your dance. You get to pick up a new way of doing your basic, a new flavour to ad to your dancing, a new tool to create yout OWN flava !