OTOH, you will be wasting that promotion, and you will still have to reach the full experience value for another level to get another promotion for the unit. "Saving" a promotion to waste this way is not a good idea - use the promotion to avoid taking the damage instead.
A valid reason to save a promotion would be if you don't know what you will be facing yet - for the early promotions, what terrain you will be fighting on. But also remember that the combination of huge penalty on defense in open terrain and nice bonus on rough terrain, means that most fights will be on rough terrain. (Also note that attacking a city does not count as either type of terrain for purpose of promotions that count.)
A valid reason for using a promotion on "full heal" would be that otherwise the unit is doomed anyway. You will have gimped the future growth of the unit, but that's better than losing it.