I've never had to deal with police once on the road in a different country.
I have been asked to produce my IDP before being allowed to pick up a rental car arranged in advance.
I don't find the cost or inconvenience of getting an IDP prohibitive.
I suspect that I would find the potential consequences of NOT having an IDP when required far more inconvenient.
YMMV.

1) We have rented a car thru Gemut.com; it's based in Oregon, started out 30 years ago specializing in German tours, since is more general.
2) They're thorough, knowledgeable; in 7 rentals, never had any misunderstanding about tolls, IDP, vignettes, etc. Eliminates all the uncertainty. I suggest you use them.
3) Had a substantial ($4000) accident in Slovakia (Slovak woman passed me on a 2 lane road going around a curve up a mountain, to avoid a headon, she smashed me instead). Slovak cop asked for my papers, preferred my American drivers license, gave breathalyzer tests, I drew an excellent accident diagram to avoid language barrier. He called for help, another officer came (who'd been a trainee in Pittsburgh, would you believe) who spoke English. They fined her 1000 koruna on the spot ($300?), me nothing.
4) I suggest you use Google Translate in advance to learn some basic words each language: Thank you,.....no parking.....no passing,.....speed limit etc.

Just my two cents: Where I live, it's easy and cheap to get an IDP. Although my husband does all of the driving on our trips, I always get an IDP on the remote chance he won't be able to drive at some point and I'll have to get behind he wheel. I put this in the same what-if category as each of us carrying different credit cards on the remote chance one of us gets pickpocketed.

Another vote for gemut.com -- very helpful!

5Alive,
You know I especially mentioned the Russians or Chinese, because they are... well, how to be PC, the "risk factor group" for the Italian (and some other EU countries police), so their chances of being stopped are quite good. At the same time chances of a Norther American tourist being stopped are close to Zero!!! Even if they stop an American, they will let him go the very moment they see his passport.
That;'s why for quite a few people this IDP is an absolute waste of time.
Tomboys example of the Police being mostly interested in his American driver's license is just another proof of this theory

Again, sorry I brought this story up. The point was that working with AutoEurope, the answers were a little inconsistent with the actual agency rep.

All the tolls are available on this website : Highway toll and vignette in Europe - Tolls.eu