I would argue that the closer you can crush the grain to when you dough-in, the better. Crushed grain is subject to oxidation of malt flavors, period. If you're aiming for the best beer you can brew, you don't want grain to be crushed and wait days or (shudder) weeks before brewing with it.
I do some of the tenets of low oxygen brewing, one of which is crushing the grain right before doughing in. You can even go further (which I usually do) by putting the crushed grain in the mash tun, running CO2 in via the ball valve and below the false bottom (to clear as much air out as possible), and then underlet (meaning fill the grain-filled mash tun from the bottom) with water I've pre-boiled and then cooled to strike temp to remove oxygen from the water.
That's way more than most will do, but we're still back to doing the best we can, and crushing the grain days or weeks before using it is not optimal.
That, btw, is not the best reason you should invest in a mill. When you invest in a mill, you control the crush, not some LHBS whose interest might be different from yours.
So, get a mill. A good mid-range mill is the Cereal Killer, which you can get for $99 including shipping. Add a decent drill to power it, and voila! You have control of your crush--plus, you can start thinking about buying grain in bulk and storing it at home. You can, with a little planning, eliminate most trips to the LHBS, plus buying grain in bulk costs less.
Get it.
The above is my 2 cents.