From the Arizona Master Gardener's Manual, Pruning trees in general: (nothing specifically about mango)

Reasons for pruning:
- stimulates shoot growth by reducing the number of buds available for growth
- it's a dwarfing process and can be used to control tree size
- improves tree structure
- thins the crop resulting in better fruit quality

The best time to prune is during the dormant season. This is usually during the first several months of the year. The structure of the tree is readily identifiable and decisions are much easier.

Fruit trees should be pruned annually starting at transplanting. There are three stages of pruning during the life of a fruit tree. 1) transplanting 2) training; and 3) mature. In order to correctly prune these trees in all stages there are certain factors and characteristics related to each kind of fruit - desired shape and fruiting habit. The shape preferred by a kind of fruit will fit into 2 basic categories: 1) central or modified leader or 2) open or vase shape.

Basically all fruit trees can be trained to an open center with judicious prining. However, the normal growth habit of some kinds of fruit allow easier training to the central or modified leader system. Also a tree trained to a central leader will take less space.

The training process starts immediately after planting. If the new tree is an unbranched whip, cut it back to 30 inches above the ground. This will remove 1/3 to 1/2 of the tree depending on size. If the tree has branches, select up to four that form wide angles (45° or larger) with the trunk. Space about 4 - 6 inches aprt on the trunk in a spiral arrangement. Prune these back to 1/4 of their length and prune the top to 12 inches above top branch for development into a cientral or modified leader. For an open center or vase shape, prune top off just above the top branch.

Also Googled 'when to trim mango trees' and got this page of great info - see the link.

HTH.
Mary

Here is a link that might be useful: Mango Trees

I don't know myself, but you might look up the phone number for Tropical Mangos nursery in the phone book, and ask them. :)