I like making them online because then I'm not tied to being on the phone.
The online system rolls to the new day at 6:00 am ET and the phone lines open up at 7:00 am ET. So, I would start with the online system at 6:00 am ET first and then if you prefer the phone, switch that at 7:00 am.
Just note that even if you're online right at 6:00 am, some restaurants may already be booked up (e.g., BOG). Other on-site guest have had access to that day for up to 10-days already (because of the 180+10). So, if you're an onsite guest, you're best bet is to start with the end of you trip because fewer people have already had access to those days. Also, even if you don't get all of your desired ADRs on you 180-day, don't give up! Keep looking for ADRs...people's plan change and ADRs come open at unexpected moments. You have to be persistent (and a little lucky), but keep trying to get the ADR you want right up until when you leave for WDW.