Ninni...pruning is always desirable, and can be very beneficial to fruit
trees and fruit production. Pruning is my main profession, and the 1st rule I keep in mind in making a cut is, "what reason do I have to remove this?"
Dead wood should always be removed. Crossing, rubbing growth, weak growth, cracked/broken wood, of course diseased wood, should all be removed. Suckers should be removed unless you want to train them into desirable growth. Thinning is often desirable. Fruit
trees in particular need some pruning to reduce the weight of the fruit load and possible breakage.
As for topping, it is not recommended as a routine practice, BUT...there are many reasons you might want to do it. Many orchards routinely "sheer" fruit
trees to redirect energy and facilitate harvest. If you want to reduce the size of these
trees, crown reduction (Appropriate topping, as opposed to "dehorning," indiscriminate cutting of main trunks) can be done, but keep in mind that it may leave wounds that never properly "heal." Please feel free to ask me any questions you want to...I realize this may all be a bit much information, and may be confusing. I'd be glad to help all I can!