If people are willing to give you an interest free loan it's probably a decent indicator that you could charge more per month.

Since cash is vitally important for startups you should certainly encourage as much up front as possible. It probably makes sense to use the "behavioural trick" of having an offer like X per month, Y<(12*X) per year, Z=Y+free month per year where Z/13 is what you intended to charge per month in the first place to nudge customers in the right direction (even though I'm somewhat meh on the idea and would personally prefer very simple options with no trickery)

As an aside it's a great validator for your business model if a lot of people are willing to pay a year up front.