2.15 wide rims front and rear should be just fine.
Per the Avon AM26 Roadrider fitment chart (link below), a 2.15" rim width is recommended for a 90/90-18 front, and a 2.50" rim is recommended for a 100/90-18 rear (2.15" is the recommended minimum).
http://www.avon-tyres.co.uk/motorcycle/?page=tyres&method=showspecs&id=6Many folks run these tires on the very narrow 1.6" and 1.85" OEM rims with no ill effects, but going with the modern tire manufacturer's recommended rim widths will provide superior performance.
On the other hand, if you're thinking about the wider rims in order to mount oversize tires, please search for any of the countless threads discussing the subject, and then stick with the OEM equivalent sizes.