There are a variety of factors you need to consider including your current year salary and your future plans in terms of returning to Canada. For example, if you had high income the year you left, you may still want to make an RRSP contribution in order to reduce your taxes owing.
Withdrawing your RRSP is not mandatory once you've become a resident but it is recommended if you do not plan on returning to Canada since special forms need to be filed every year in order to defer taxes owing on the RRSP in the US which may be a cumbersome process should you not have the appropriate accountant.
