I'd say it's a good idea to fly with different instructors to either
find someone that you get along with better or to occassionally get
some new input. The only important point is that you do maintain one
CFI as your main instructor to monitor your progress.
I changed instructors at around 10 hours and since then took lessons
from mostly the same CFI but also did some flights with 2 other
instructors (mostly for their different takes on improving landings).

- Marco