It may be worth using some extra Maleficent coins to buy tokens. Since the tower challenges have such horrible gem conversions (5,000 : 1 gem, historically), I'd suggest it's certainly worth spending however many coins that might otherwise be wasted. I believe - not certain though - that the system will round up your coins, so it would be worth at least spending any coins between the range of 2,501-7,500 down to close to 2,500 (and so on) - you'd only get one gem in that entire range regardless. I've actually been trying to decide whether to just spend all my coins stocking up on tokens, compared to the 3-4 gems I would otherwise get, but that number of tokens might take a week or two to collect, so 3-4 gems are worth it to me, I think.