It kind of is. I dont know that "safe" is necessarily a criteria... i don't think connecting to a relay exposes anything other than the notes you send to it. And all notes posted to nostr should be done assuming it's for public consumption anyways.

Generally bigger relays will have more connected users providing further reach (caveat outbox model per derek). I think its more important checking the uptime of a particular relay and if there are any/many errors. I know you can see this info in amythest. Not sure abt any other client.