I've done it both ways. At home and out to a shop. If it's cheap enough for a rebuild on a head you carry in, I think it's cost effective to do so. Providing it's quality work, of course. If you can check/inspect the other valves, it probably isn't bad to do just the one valve, lap it in, and reassemble. Bottom line is whether all the other valves are holding compression, and since you'll have that one valve out, the condition of that valve seat. It would be nice if you had a spring compressor that fits, and a spring tension tester. Then you'd know for sure what the condition is, and would be able to see all the valve seats and parts and judge them. Short cut --> I'm old fashioned, and will pore gasoline down the inlets/outlets and let it sit for a bit, to see if it leaks into the combustion chamber (with the head off). It shouldn't leak through at all. If your head isn't a leaker, fix the one valve only. My opinion; Japanese springs (all of them) aren't the best, and whenever possible, I'll replace them all with aftermarket of better quality. Maybe Japanese springs are better now than they used to be, but I don't know that. Of course, some aftermarket springs will be rough on the camshafts and rockers.