I pretty much agree with Mark when it comes to observing globular clusters for the observing pin, that having the same eyepiece which is required if I recall. However conditions can change from night to night so having the same power in the scope for each object is really just one easily controllable variable.

For example I observed G1 several times, and only had one night that was really ideal for spotting the little cluster with my scope. Obviously with a larger (or smaller) scope the conditions needed could be different.