Can't be worse than checking for collision for every unit every tick though ! <img src="smileys/smiley17.gif" border="0" align="middle" />
In fact, if you use only a "sum of deltas" approach instead of checking for the exact distance, I think it would be much LESS expensive than collision checking. Wouldn't hurt to try ! <img src="smileys/smiley2.gif" border="0" align="middle" />