If the car:

1. Is available.

2. If the car rental company doesn't pull shenanigans such as cancelling your rental at the last moment due to <reasons>.

3. Is the kind of model you want or need.

4. Has the extras you like/need (adaptive cruise control, etc).

Etc.

In my experience car rentals suck, I wouldn't want to use them except for when I'm flying somewhere.