User year only matters when it comes to banking and cancelling deadlines. The idea is that if you have to cancel a trip, you want those points to have a decent amount of time remaining in theri UY in order to either bank them or use them on another trip.
Therefore, it is generally wise to get a use year just before the month the you anticipate traveling the most, if possible.
I would think it is best to keep the same UY for all your subsequent contracts if you can. Keeping track of different Use Years and banking deadlines would get kinda crazy with multiple contracts.