no need
but i suppose it depends on rental company, i have never needed to do anything
It's not the rental company you have to worry about - the big ones won't care, especially if you already have a 'record' with them back home. They type your name into their computer, pull up your record and know they can get your money.
Avis in France and the UK haven't even asked to see my licence - they put my name into the computer and all my details appear, including my drivers licence number. The most i've done is show them my avis 'wizard' card so I don't have to keep repeating the spelling of my name. They just type the 'wizard' code into the computer any my details appear.
On the other hand if you are involved in an accident or get pulled over by the traffic police for what ever reason, it might be a different story.
There tends to be a mesh of bi-lateral agreements between countries, so it does vary.
An IDP is strongly recommended in non English speaking countries as there is no guarantee the traffic police would be able to read English, thus even know the ID you are showing them is a drivers license. The IDP document is in a standard format (and multiple languages) that all traffic police are supposed to know about.
The AA people probably have a table that lists by country if a IDP is 'not needed', 'recomended', or 'required'.
Even in the UK, it may be 'recomended' as if you get pulled over by the police in a rual area by a local copper who's never been out of the country, he might not recognise your license as a license, it could be a bank card with photo on it for all he knows.
I did get told a funny story by a collegue who was involved in a junor rugby 'tour' years ago - a number of the boys were under 21 or even 18 and didn't have any drivers license. So they could all go out drinking together in states in the US were the legal drinking age is 21 they made up a set of fake 'drivers club of Australia' ID cards. (There is/was no such organisation). It was intended that this would only get them into drinking establishments.
My collegue was horrified to find out that the boys used these fake IDs to hire cars in Hiawaii. Some of them didn't even have drivers licenses back home. It would appear the hire company employees thought they were driving licenses.