Our corporate policy allows us to use our corporate rate when traveling on leisure. It's in our internal t&c's and we have a letter from marriott corporate sales management. They simply request that we indicate non-business trip as personal or leisure when reserving a room or when asked by FD personnel.

The only time I use another corporate rate (that of my client), is when THEY book the travel on my behalf (in which case their corporate travel policy is in effect). They almost always include a letter and temp ID I can use for the rental cars and hotels (if not, a quick call fixes it).