You can, and should, make dining reservations as soon as possible, with or without a resort reservation.
Anyone is welcome to make dining reservations up to 180 days in advance. You don't need a resort confirmation number.
You don't even need to be staying on WDW property.
However, there is some benefit to having a confirmation number in hand:
The advantage to having your WDW resort reservation number is that, in most cases, you can uses the
180 + 10 rule. This means that guests staying at Disney-owned resorts can make the meal reservations
for their entire trip
(up to 10 days) on the first day of their 180 day window. So instead of
having to call back multiple times, a Disney resort guest can call at
the 180-day mark to make reservations for not only meals on the first
day of their trip, but also for
meals on the following nine days. In reality this means that Disney
guests who are staying for 10 days can make the reservations for the
last day of their trip 190 days in advance, rather than just 180. This
gives them a boost in their ability to get reservations for
hard to come by meals.
If you are making your dining reservations online, you can begin at
6:00 a.m. If you're making your dining reservations over the phone, the
lines open at 7:00 a.m. at 407-WDW-DINE. Either method works 7 days a
week.