Is it ok to paint walls now and doors and casing later?
Other than cost, which will depend on the painter, there isn't that much of a difference. The concerns I know of are:


Matching Paint Colors
That is (a) not nearly as much of an issue with white as with other colors and (b) if you are talking about a short amount of time (weeks as opposed to months or years), you could get all the paint at one time.
Aging
Painting part of a room 2 years after painting another part of the room may not match exactly due to aging effects from sunlight, etc. But a few weeks, even a few months, is not likely to cause a noticeable difference.