getting change from the first toll is a good idea. But be advised that they won't take anything larger than a $20 bill
the first toll on 528 (north exit on the airport loop) is manned
i think the first one on 417 (south exit on the airport loop) is also manned, but I can't be 100% positive since they have redone that interchange recently
You could also get change at the airport by buying a cheap thing from a store (getting a magic band at the disney shop if you're staying offsite is an option) but it's rather exepensive in the grand scheme of things, because there is a tax surcharge of 10% on any transaction at the airport
a $20 at the first toll is the best option
then you can also consider your car hire tollpass policy
if you're renting from alamo, you can use the sunpass lanes, you'll be charged $3.95 per 24hrs periods where you actually use a toll + the value of the toll
it's a bit more expensive, but you don't have to slow down, or worry about cash
I wouldn't advise it for other rental companies (
avis would cost $19.75 for the duration of your rental, if you hit one toll + the toll values, and dollar would hit you with $50 a week ... so stay clear of them)
check your rental company's policies re:tolls