What brand and model? Blackburn lookalikes (ex.: MEC) are often crooked, but I can tell you that my two Burce Gordon are very straight and very rigid. So the rack may be crooked, but there might be other issues:

– fork is crooked, so the wheel doesn't sit squarely in the fork ;
– wheel is off-centred (dished improperly).. You may check the dish with a proper tool like this; it may also be measured on a table.

You may want to rule out these factors by trying the rack(s) or the wheel on another bike.