Ordering now less will be sold out, and they will ship in the spring. If you are in an area where freezing weather hits they will avoid shipping when they could freeze in the mail. By April many will be gone but there's lots of nice stuff out there, so look it over during the winter and I'd order by the end of Jan. if you have particular things in mind.
That's just my 2 cents worth.

Jamlover is right: order now if you want to get your reservation in for popular varieties. Dahlia growers will not ship til appropriate for your zone and by waiting you risk them being out of stock.

Order now, ordering later brings only one thing: SOLD OUT. Growers usually ship when the risk of the tubers getting frozen is over.