I always telephone the night before, if its raining really hard the next day the customer knows I will not be turning up that day.
If the rains stops and I turn up and the gate is locked, I just clean the front, leave a ticket and clean the back at the next clean.
I know many customers who have complained about cleaners climbing over their gates or roofs and the damage that is left behind, mainly from broken or displaced guttering.
Don't forget if you cause damage, you are liable, and how hard is it going to be for you to prove that the loose or broken guttering was already there, if you haven't climbed over it you cannot break it.