Generally speaking, sooner is better than later.
Not getting a crown can result in the tooth fracturing, and a crown might not be possible then. Also, if the tooth with the root canal is visible when you smile, it may turn dark and diminish your smile. A crown will prevent that.
Waiting a few weeks before getting the crown is not always a bad idea, though. Allowing some time to pass will determine that the root canal was successful before investing in an expensive crown that might be lost if the tooth has to be extracted.