Bicycle Tutor

Is there a real need to service pedals (i.e. clean and grease them, replace bearings), or are they generally just replaced if problems arise?
Any tips on freeing potentially seized pedals?

It's a cost/time calculation for the most part. Most lower end pedal are not really designed to be serviced and are treated as disposable. Mid and higher end should be cleaned and repacked occasionally. But they are often a bit trickier than repacking a hub because it's sometimes a little harder to open them up and they can have lots of small bearings. I don't think you can replace the bearing surfaces on most pedals, just the bearings. So once the races are worn, it's generally time to replace. Many better clipless pedals use sealed bearings so they last a long time. But it comes down to what you have. I say go for it, I hate to see good parts get treated as consumer disposable items for no reason.
It a pedal won't rotate anymore, there's probably a crushed bearing inside that wedged in. Unlikely this could be fixed even if it was worth the time to work on it.

Thanks for the suggestions. As for "seized", I meant bolted into the crank, but rusted/stuck there a while, whatever.
http://bicycletutor.com/replace-pedals/
Would just forcing them with a 15mm wrench coupled with a pipe (maybe) not damage anything? Is there a way to get WD-40 or the like onto the threads?

Using a pipe for more leverage is a time tested technique! The risks are damaging the tool, rounding out the flat on the pedal, banging your knuckles, etc. Pretty unlikely you can hurt the crank UNLESS you're turning the wrong way. Remember that the left pedal has reverse threads. Also be careful about how you brace the crank. If you're pushing against a chainring with that much force, you'll bend them.
I've never had much luck with chemical approaches for seized parts though others claim to have. I think the chemical can't really penetrate far enough to make much difference. If it is a steel crank, then a good rust breaker may help a little.
Worst case: I've had to drill out a pedal from the crank before. You drill from the back side. The tricky part is to drill out enough of the pedal spindle without hitting the crank at all and damaging the threads. You don't have to completely remove all the material from the pedal. Sometimes if you drill out most of the metal, it's grip in the crank weakens and you can get it out by unscrewing then. Just be careful, easy to ruin the crank arm.

My LBS-guru did the following:
- move crank arms to horizontal position
- take 1 big hammer, place it under the crank under the stuck pedal
- take 2nd big hammer, hit crank on top of stuck pedal
- take pedal wrench, place on pedal, carefully tap end with hammer (both directions)
repeat, if necessary.
Sounds brutal (and it _is_ brutal), only advisable on older parts, in my opinion. I don't want to know what this does to new cranks...
Caveat: when applying too much force on the pedal wrench, you can strip out the crank arm's threads! This is especially true, if somebody installed the pedals without grease...

I have clipless double sided shimano pedals for my bike. I put some oil in the pedals about 3-4 times a year and have not had a single issue and the pedals are 10 years old now. I have no idea if they are sealed units however. I'll just keep adding a some oil now and then and keep riding.