There are two answers to this.
No you don't. The crews will complain and their morale will drop and this doesn't affect the ship at all at least in 6.1 (and I don't think in 6.2 where I think that morale for ships that are unaffected by morale is removed). Principly you could reduce the time even below 3 months.
Yes you do. The crews will complain and their morale will drop and if you are role playing this is/can be a big issue for you. Not to mention the constant complaints that show up in the event list.
So the real answer is that is up to you if you do or don't. I sent crews on ships designed for 6 months on journeys twice that...but the first chance I got I upped their crew endurance.