Our road regulations are mostly downloaded from Europe.
"Recommended" in the US is "Approved" tyre width here, and I believe similar in Europe.
Therefore, the manufacturer approved width for each tyre is supposed to be followed.
I gather that the wall of the tyre is supposed to be straight up and down.
Blockley Tyre on their web site says:
"It is designed to correctly fit rim widths available in period (typically 3 inches for 500 and 2.5 inches for 4.50 sections). Presently the higher performance road cars fitted with, say, a 5.00x19 race tyre have had to be fitted with a rim measuring at least 4 inches across the well as recommended by the manufacturer - A ludicrous situation as cars did not have such wide rims originally. As an example G.P Bugattis, with their aluminium wheels having a width of only 2.7 inches either have to fit these same tyres, or fit non-standard wheels - either solution being unsatisfactory."
http://www.blockleytyre.com/page2.htm
"A friend of mine owns the Zagari photographic archive and by studying the photographs, coupled with checking period data, I think I can safely say that no competition car pre-war had a rim width exceeding 4 inches, which may come as a bit of a surprise to most of us. My just pre-war GP Alfa Romeo only had a 4-inch rim and that was for their widest 7.00 section tyre! I therefore surmised that if one were involved in a big and expensive enough accident an insurer could argue that the Dunlop tyres being used were too large for pre-war rim widths, information which is clearly stipulated in their data. Conversely if wider wheels were fitted to accommodate these inappropriate tyres (as so many have had to do) it could be pointed out that the vehicle has been modified out of specification -and rightly so because wider wheels tend to be built away from the brake drum which is why one often sees them laced on the inside and middle only, widening the track, altering steering geometry and so on and, worst of all in my view, making the cars look like out of character 'hot rods' which we have begun to take as the norm."
http://www.blockleytyre.com/page4.htm