I guess it depends what type of service you're looking for. Removing the lowers and changing the seals and oil isn't too bad of a job, but it's not really going to improve the function of the fork that much (unless they are seriously dried out). That is basically just keeping the stanchions lubed and preventing dirt from wearing them. If the fork is working well, then I would recommend that at the least. If you want the fork to feel like new, you'll want to service the uppers, which is more involved. I think for most forks, the lowers should be serviced every year, uppers every 2 or 3 depending how much you ride and how picky you are about the feel of your fork.