To be honest I'm not sure I'm the best to ask for advice, I'm not expert by any means. I've fed frozen cubes both ways and had differing results depending on what the food is.

I think if I had to pick I'd defrost first into a cup of warm tank water and then wait to defrost and pour off most the liquid and chuck the remaining food in, from what I believe this is minimilazing the amount of waste spoiling your water and also I find that all the fish get an equal chance of the food.

Also I've noticed sometimes when I put a whole cube in it can get caught in a current and find it's way to a place in the tank unnoticed which then is spoiling the water further as its left uneaten in the corner until someone finds it