I like to consider myself a little bit above the average skill level when it comes to melee (unless compared to San or some other weenie pro) so on a personal level I love to duel the last guy alive even if he has a high chance of winning. While there have been times where my team had 3-4 players left and we dueled there last guy only to all lose 1 at a time, there have also been times where I have won 1v4s and nothing is more fun and feels better.
Personal preference really. If you want to gank the guy to ensure victory, go ahead.