Things that require an alignment: (please add to what I miss).
- Torsion bar adjustment
- Front end lift (replacement of a-arms, etc)
- Rear suspension adjustments possibly (Add-A-Leafs, Shackles).
- Offroading (if you offroad, alignments are required more often)
- If you truck pulls consistently in one direction on even pavement
- New wheels might if they are of different dimensions than the stock wheels (not 100% sure on this).
- Replacement of tie rods (either broken, or going to heavy duty ones)
Different size tires don't affect the geometry of your wheel, so you can switch between different sizes without needing an alignment
You should be able to get an alignment for about $40 (at least in the NW anyway). Also check to see how long they warranty their work if they don't align it right (minimum should be 30 days, some places go longer).