I walk on some sloping roofs, but only if i am sure i will be ok. If there is any doubt in my mind if it is safe, i just tell the customer, i will not be able to clean that window incase i fall. Everybody is oik with it.
If you work with someone else, you can get around most problems by being footed.
Also make sure you stand on the bottom of the tiles, where the two tiles meet. That way you have very little chance of cracking any