For new and aspiring models it is important to think about this topic carefully. If you are under 18 years of age in the United States, you must have a parent/guardian present at all times, whether it's a casting, photoshoot, interview, whatever. It's the law. However, if you are 18 or older, then it's your call whether you want to bring an escort to your shoots (they won't be allowed at castings or interviews with you most times--although they may wait in a waiting room if one is available or in the car).
In terms of shooting with a photographer, you may run into one of these two situations:
1) The photographer will be totally cool with you bringing an escort and may even encourage it.
2) The photographer has a "no escort" policy no matter what (with the exception of underage models if they're smart)
If you are allowed to bring an escort, please choose wisely. For obvious reasons, it is not best to bring your boyfriend, fiance, husband or girlfriend, fiancee, wife. There are too many reasons to name why choosing your significant other as your escort is bad but here are a couple:
1) They can be distracting to you and the photographer
2) You may get shy or nervous by having him/her present on the shoot
3) He/she may get jealous, upset and may throw an emotional tantrum
4) He/she may end up trying to tell the photographer what to do and how to shoot you
All these add up to a bad working environment and experience. It is best to bring a best friend of the same sex or at least a friend of the opposite sex that you have a strictly platonic relationship with (this is where having gay friends comes in handy lol).
Each photographer feels differently about the idea of bringing an escort if you are of age so take it on a case-by-case basis.