Other Car Rental Programs (ie. Alamo, Enterprise) - Know I will be 2-3 late - should I reserve car that way or take chances?

The way my trip is planned I will have 2-3 extra hours on the rental. Are there any savings to reserving these 2-3 hours in my original reservation or should I just book in 24/hr increments and pay whatever extra I use?

Bottom line is the extra hourly rate different if its pre-booked? (i.e $20/hr if planned for, but $30/hr if not)

Not that I know of. Here's a kick: Normally, the hourly rate = 1/2 the daily rate. So, after two hours, you've bought the whole day anyway.

Depending on what city you are in, I've had small success with this: If it's 3 p.m., tell the person who is checking you in that you kinda got lost and could they check you in at 1 p.m. Slip them a few bucks and they may do it. Someone did it for me once and it didn't seem too difficult to change the time on their machine.