When to reclaim and when to repair

You might want to check: http://zero-k.info/mediawiki/Repair

One observation: reclaim does not use any resources, repair uses energy (so, if you have no energy, don't repair). As I generally lack metal and have energy I mostly prefer to reclaim if available (unless specific circumstances).

I don't understand this question....
You repair when units are alive and damaged.
You reclaim when they are dead.

It's always worth repairing units since it's cheaper to repair than rebuild.
Even if it weren't, a damaged unit can still inflict damage if needed. A new unit being built, can't.

Caring for and repairing your damaged units, will eventually result in a bigger army to crush the enemy with.

My Smartest Builders widget implements a good approximation of the optimal approach: It prioritizes repairing over reclaiming except if you have energy deficit. If you have metal (or energy) excess, it only reclaims metal (or energy) if allies have free metal (or energy) storage.
Determining when there is a resource deficit or excess is the tricky part: How a constructor that is controlled by my widget sees the resource situation depends on its buildpower. That units can also heal themselves is neglected.