As an unplanned, random encounter, yes, as a potential hireling, typically no, but...if the NPC is being sent to the person looking to hire them from someone or somewhere (ie, guild, church, etc) that they trust, then very rarely would there be any adverse factors of concern. If the NPC is coming to them via a posted ad at a local establishment, then anything could happen. In either case, that doesn't mean that they wouldn't have an unfriendly reaction, where you know that they have a good alignment, and maybe there might be some evil aligned character(s) in the group (or vice versa) that might 'rub them the wrong way' or the group is comprised of unusual race types that might illicit some degree of racism or rude type behavior (jabs, jokes, etc). The types of traits/characteristics of the NPC should be rolled up before-hand and depending upon the variables in the situation, it would require a logical case-by-case judgement call if need be. Just don't give the players all the NPC stats, like alignment, keep some fun for yourself ! Just because the player is running the NPC doesn't mean you can't interject some of their dialog in situations that demand it. They are people too, not just a stat sheet, so flesh them out with habits (good and bad), quirks, tendencies, likes, dislikes, etc. The NPC is a vehicle by which you can communicate with your group. Don't lose sight of all their purposes, other than an extra attack roll for the players in combat, otherwise your sessions will not be as enjoyable as they should be...a good balance of roll-playing and role-playing. Without NPCs an RPG is not an RPG, it will be boring.