When should I retreat a unit in CoH?

Normally I would retreat a unit just before it is about to die, or when it encounters an enemy which it cannot counter and cannot reasonably evade.

Times when I might not retreat as above might be:

Keeping the unit in place to take enemy fire

• allows another unit to flank.
• stops an enemy from helping another battle elsewhere.
• keeps an enemy still while artillery is about to obliterate it
• keeps an enemy tank in range while AT is brought into range
• waiting for a unit to get an upgrade (SMG, shrek/recoilless rifle upgrade)

Apart from special situations like those above, and all other factors being equal I would normally retreat a unit so it just makes it back to base with minimal health. This is usually with one or two men left of a squad.

Be careful about being too quick to retreat, if possible just walk away/around if you run into a unit in a building or long range unit in cover.