GraphicGr8s· Pixel Prestidigitator
What is the idea behind never topping a tree? Citrus is regularly topped as are many others. I top certain trees to break the topical dominance and get it to bush out. Although there are certain trees that shouldn't be pruned when sap is running most of the others you are in control. Some times of the year are of course more desirable. You don't want to prune fore instance when a tree is in bud. It reduces your flowers. But will it hurt the tree? No. So long as you make the cuts properly.1) Don't top a Maple, or any tree for that matter, ever.
2) Don't prune it in spring, Maples bleed a lot and pruning at the wrong time of year can kill an already weakened tree. Late summer (after it has leafed out fully) or later winter is best.
Honestly its not looking so good, hard to tell but did the top die back already? Personally I would make sure it gets lots of food and water this summer and then think about pruning at the right time and when the tree will handle it better.
Some pruning is done to shape the tree. Some to get rid of a future problem like a crossing branch or a bad crotch that will be susceptible to breaking with a heavy wind or snow load.