I'd recommend at least a level II holster. As stated before, you never know what someone else is thinking, and what they will do. With just a regular level I holster you won't have enough time to react. People always say oh I keep my guard up, etc. etc. but 9 times out of 10, action will beat reaction. In the police academy I borrowed a friends blackhawk serpa level 3 holster with a glock 35, and someone who knows how to work a holster can either get it out completely, or get in a decent fight for your life before you know it. Someone behind me could snatch my glock out of a blackhawk level 3 holster before I could react fast enough.
Just food for thought