They will look at what your competitors pay for "parking" and set a benchmark. If you are out of the range they are used to seeing they will want to see some kind of documentation.
99% of IRS "compliance" audits are triggered by something the computer sees as being out of the normal range. If your peers are paying $5 a day to park in a garage or plug the all day meters and you do as well playing the "dodge the meter maid" game on short term meters you probably get away with it.
I would still have your drivers making out a daily log and spreading their parking tickets out across the days, if for no other reason, so you know how much it is costing you.
When I was on the road working I kept a log of everything I did in my vehicles and it really came in handy later. The trick is just getting started doing it. Once it becomes your routine it is just second nature to write everything down.
