All depends on how you want to market it to your client really. There are benefits and drawbacks to both methods, so you could choose which works best for you or give your client the option.
For example, I offer hosting for my clients and they can choose either monthly billing or one yearly billing with a small discount for paying for the year up front. 9/10 of my clients choose this option ... some may be for the discount, but when I explain that their domain is registered on a yearly basis, your hosting would make the most sense to do that as well. Since it's not a huge up-front cost, that helps too.
When it comes to a maintenance agreement, how you offer up the services will factor in the monthly / yearly decision as well. Do your hours overflow each month if they are not used from the previous month? If they do not, the concept of paying once a year but only getting X hours per month may not be one your clients easily understand (and agree with). You would probably need to adjust it to be a yearly amount of hours available.
As Mike mentioned, there is also the freedom of a client knowing that if they decide to change their minds, they are not "locked in" to a contract. If they aren't doing much maintenance, they can cancel for the following month and feel good with your business still to come back later if needed. If they had to continue to ride out the year and pay for time they don't use, they may cancel at the end and definitely not return for changes down the road.
If the monthly system you are using is working for you and your clients, I wouldn't worry about changing anything simply because other businesses are doing it different. Stick with what works for you.