I use a pair of vise grips on the end of the axle stub with a piece of rag over it and gingerly bend it back until I'm visually satisfied then stick a wheel on to further examine it for satisfactory alignment. It's not a car, so unless it's obviously scraping our steering the mower, it's hard to go wrong.
Don't forget the grain of the lawn can steer the mower too so a visual inspection will suffice.
