i think it's really important. not only from a trust perspective, but also the amount of time you are going to spend with them means its vital you can have a laugh together when things get a bit tough. the risk is that you may fall out over something related to the business - this is something only you can decide, but if you address the possible scenarios early on and have agreed upon a way of dealing with possible outcomes (good and bad), then i think this risk can be limited.