It depends on your budget and plans.
I like to buy later in the season, but I have no Cup hope, and that's a fact. This season my only goal is to do well in League..
I spend 1 Token for 5 star high quality player.. I hate bidding wars, and the auction system bores me. I also buy my youth if they are really good, but only before half of season.

If it is a player you plan on keeping multiple seasons, that is to say, a fast trainer, I don't think a week or two makes so much difference, and at this point in the season you may be looking to replace guys that are just playing bad. But I do get as many greens as I can and train the young guys to keep them.