Anyway, philosophies change at various times but at the time of WWII for the most part I think it was felt that new troops should join seasoned troops and fill out their units so they could "learn enough to stay alive" and become seasoned troops. The alternative is to reform decimated units to create new units of troops that have all been in theater about the same length of time and let the new troops form their own units.
The result would depend on a number of factors and a good commander might stay flexible in this regard. If the "old troops" were pretty beat and starting to "resent things" it might not be good to rely too heavily on them, especially if one had an ample supply of "young lions" to throw into the fight. So it might not automatically be best to spread the new guys around, especially if what they learned was a bad attitude.
In Korea all the troops were pretty fresh but the decision had been made in the interval that concentrating "elite troops" was a bad idea and so there were no Rangers and few Airborne. I suppose the commanders of the day felt that Mech. Infantry were all "shock troops" but in reality it didn't work. I would think a good commander would want an "Ace in the hole", a unit to throw at the enemy to knock him on his heels and so we have all these Rangers and Delta, etc.