This question builds on the hypothesis that people in the front row in a melee battle were very likely to die. First, is this assumption correct? If so, for an individual point of view it was not very desirable to be in the front row but in a strategic point of view, having the best warriors in the front would more sense (I assume).
I am also interested in battles of ancient Rome or Greece if that is your expertise. Probably the answer to the question varies from point in time and how important the first row was for an army setup vs the honor involved of dying in battle of a particular culture (?)
This is certainly only a partial answer, but my impression is that certainly in the central middle ages (c.900-1200) that the best and bravest soldiers were expected to be in the front line of battle. We know this because when chroniclers want to emphasize how awesome some is, they'll have them fight in the frontlines. The biographer of Bishop Gerannus of Auxerre (910-14) has a wonderful juxtaposition of the selfless bishop leading his own small retinue out to fight the raiding viking army, while the evil viscount of the city (boo-hiss!) watches from behind the city walls. Similarly, Godfrey of Bouillon, first Latin ruler of Jerusalem (1099-1100), developed a reputation during the First Crusade, certainly amongst his own Lotharingian and German troops, of being an excellent fighter who was always the first to charge the enemy. Despite not being the brilliant tactical commander that some of the other crusaders were (e.g. Bohemund), his reputation for personal bravery seems to have secured him something resembling universal respect amongst the notoriously bitchy crusade leadership.
This, of course, is the theory. I can't say with any certainly that the best soldiers actually were in the forefront of the fighting, but contemporaries certainly thought they should be.
On a side note, you should note that there weren't really any cannon-fodder in medieval armies (not least because there were no cannons!) and you certainly wouldn't find any forced soldiers such as convicts. Medieval armies were made up, on the whole, of specialists, drawn from the nobility and their entourages in the central period.