I imagine I'll get by OK with just the one. It probably could save me a little time if I had a pair. This is the first time I've ever needed to measure for length and I'm tempted to check all 16 valves just to see if anything odd shows. I'm sure we don't have any flat lobes because the motor is relatively fresh, had it's oil changed way more frequently than necessary, and there's never been any indication of debris. - But it would satisfy my curiosity and alleviate any concerns about uniformity.
One other benefit in checking all the rocker/valve clearances would be that I'd (hopefully) become adept enough repeating the process to be certain of my results. For the guys out there that have done this before, I'm sure it seems so simple that any ignoramus could do it. I'm planning on using carbon paper/tape between the valve stem and roller to see if I'm truly centered.