Curse of the shipfitter
It's always a good idea to check the condition of the wheel bearings any time the wheel is off the ground, I think.
But as far as when to do them, you have a good point that, it's already apart so why not? It is a little more involved than just the seal, but the bulk of the money in either job is in the disassembly and re-assembly. So if one is hiring it done then I'd say yes, just spring for a full service with wheel bearings, swivel pin bearings, and all the seals. He'd be paying the same labor costs all over again when it was time to do the bearings where he could get them done now for just a couple extra hours' labor.
Europa will sell you a kit of parts containig all the components required to rebuild both axle ends. Last time I bought one the kit was about $500. It's probably closer to 700 by now with exchanges what they are. But I forgot where the original poster is located. If in the UK, things seem to be more readily available, and thus less expensive, over there.