It's probably a good idea to not pay any third-party in advance. There's no need to single out people who run solo companies for the most part. Now, how about paying a company like XenForo in advance? Go for it, because they are an official first-party company!
Almost, not quite though... basically if you're paying for a continual service, then the best option is to pay monthly so you can pull the plug at any moment if the service starts to degrade.
With XenForo, its a one off payment for a product.