Many people would advise you to never disenchant troops, and always keep them and ascend them whenever you can.
Ascending troops raises their max level, and in the long run is needed to upgrade kingdom power level (stars on the map).
My advice varies a little:
- early game, below level 100-200, you most of all need a viable strong team
- so feel free to disenchant any troops below Epic rarity to make extra souls, and spend the souls levelling one or two combat-worthy teams (a strong offense team, and/or a farming team with Valkyrie in it)
- never disenchant epic or better cards, as they're much harder to get hold of
- once you are level 100+, and you have two teams at max level, then revert to above advice and never disenchant again (until you have 4+ at Mythic level of the troop, in, er, quite a long time from now)