although perhaps because almost every area I would want to live in is behind guarded gates. Is driving around looking for a rental a valid excuse to get in?
Yes, in fact the guarded gates are the best, since you just ask the guard, they always know if there is a place for rent within. A lot of times the guard's windows are covered in for rent or for sale signs.
I've even had it happen that the guard lets me know that a tenant will leave in x period of time and to check back then.
But then again, I look relatively non-threatening and have gone house hunting with my girl. If you pulled up in a '87 Hyundai with tinted windows, 3 guys in the back, blasting music and smoking a joint, you might get a different reaction.
I like to pick where I want to live first, then find a place in the area I like.
Also, in my experience, you get a good 20%+ discount compared to advertised rates. Once people go to the hassle of advertising, they understandably will have greater demand for their rental property and adjust the price accordingly.