Valve springs are not expensive and do wear out. You can reliably measure compression distance and attendant spring force at the specified distance. Jim Comstock and Dave Comeau can tell you a lot more about the metallurgy and useful life of valve springs (many posts on this forum to this point). For me it's a no-brainer, the more judgement you remove from a rebuild the better and longer lasting the results will be. If a valve spring breaks it can cause some serious damage.
I replace valves, guides, springs, keepers, insulating washers, retaining washers and seals on any head that has no history or wasn't rebuilt by someone that I know and trust. Anyone that tells me that they, simply, drive the guides out never hears from me again. A lot of services plane the gasket surface as a matter of course, they never hear from me again either.
Penny wise and Pound foolish, think about it. Replace the springs!! "Pay me now, or pay me, a lot, later. Or, better, "why didn't we have a enough time to do it right initially, but we have time to do it over"?
I wish Jim could clone himself, he is one of very few I trust to do it right the first time.
A parting thought: I think of my Nortons as poor mans Vincents; keep them maintained, in good condition and they can part of your retirement or your legacy. Keep cheaping out and your Norton begins to look a lot like a yacht (a hole in the water where you throw money).