Actual tire sizes vary a lot by tire manufacturer. Even just looking at the specs you can see that, if you look up the section widths for 275/35/18's you'll find a range of 10.5" to 10.9" just based on manufacturer, and those are all tires in the same performance category. That's a big difference if you're worried about how the tire will look on the rim. The height makes a big difference too which you can see in my pictures, a taller tire will bulge less for the same width just because it has more space to adjust to the width of the rim compared to the section and tread width.
Honestly I think anything in the recommended spec range will be ok, if you put a set of tires on the narrowest rim allowed then there will be some sidewall bulge, if you put them on the widest rim allowed they may look a little stretched. For handling the widest rim allowed is usually best, but for the street I actually like to run towards the narrower widths allowed because it gives a little sidewall protection to the lip of the rim.
Here's some pictures of my car, I run 18x9's in the front and 18x10's in the back.
This with 275/35/18's and 295/35/18's, BFG KDW2's
This is with 275/35/18's and 295/40/18's, Falken Azenis RT615K's
And again with the KDW2's
And the Falken Azenis with the 40 series 295 in the back
