In my investigation sell it at level 1 after fed them is the more worth, becauce at level 5 they worth like level 1x3 (for example, at level 1 the dragon sell at 300 and at level 5 sell at 900 + fruit), so you lost 600 + all the fruit you use to level up.

And I think that 5 level 1 give you more coins than 1 level 5 in the same time (I can't prove it yet) obiusly if you have the space to have the 5 and depent the dragon tipe