I'm a scrub and noob (playing only 4v4), but I've started to tech grenades really early in game, after my 2nd or 3rd riflemans. Grenades help a lot when dealing with MGs/garrisoned units.
That will slow your lt/capt/stuart by a mile though. Also the munitions you spend on the grenades are soo costly.. which you could've used for weapons and mines.
Buildings aren't that much of a problem to me, to rush nades.