I always use Budget because they do not put an exorbitant hold on my credit card when I rent, like say, Hertz, who I hate.
I've never been asked if I want roadside assistance. There is a big number on the key that you call if you have an accident or need roadside assistance.
I called it the other day because I thought I'd run out of gas. They said the service guy would be an hour, so I decided to chance it to the gas station and I made it, luckily.
So I cancelled my service request. I do not know if there would have been an additional charge if I had actually USED the roadside service, but I would think this is fair.
One pet peeve I have right now is that the rental companies are adding toll passes to the windshield of the car. If your car has one, they will not take it back, and they seem to rarely have cars without these nowadays. This is good if you're from out-of-town, have no cash, on an expense report or something. But the catch is that there is a surcharge for the 'privilege' when you could pay cash.
For instance, on my last monthly rental, I had an additional $63 bill - $23 in tolls/legitimate - and $40 in surcharges/not-so-cool.
An additional problem with these is that even if you pay your toll in cash, sometimes the toll pass registers the charge anyway. So if your car has one that you don't want to use, be sure to get receipts for your tolls, so you can prove you were charged twice (and surcharged) and straighten out the billing at the end of your trip.
On this last rental, I put their little box in the trunk under the spare tire. I don't believe it's registering charges, but we'll see, I suppose.
Your mileage may vary...pun intended.