The basis for getting wheel alignments done is with anything which can cause the alignment geometry to change.
In the front end of a vehicle, there are three measurements for alignment which can be affected if you do work there. There is camber, caster, & toe angle. Each are described as:
- Camber: The degree of deviation between the top edge of the tire and a true vertical line running through the center of the wheel. If the top of the tire tilts inward as seen from the front, camber is negative; if it tilts outward, camber is positive.
- Caster: The deviation in degrees between an imaginary line running through the upper and lower ball joints and a vertical line running though the center of the wheel hub to the center of the tire’s contact patch on the ground. If the imaginary line tilts to the back of the vehicle as viewed from the side, caster is positive; if it tilts forward, caster is negative.
- Toe Angle: Toe angle is the difference in track widths between the leading and trailing edges of the tires. If the track is wider at the front, the wheels are toed-out; if the track is narrower at the front, the wheels are toed in. Toe-in is used on most drag and production cars to compensate for slack in the steering components so the front wheels track straight ahead under forward thrust.
(Taken from this website.)
Anytime any of these could be moved, you need to get an alignment done to ensure things are correct. Not having proper alignment will most certainly cause excessive tire wear, but can also cause issues with steering and stopping.
Things which might affect these adjustments are:
- Control arm bushings
- Tie rods
- Ball joints
- CV Joints
- Wheel bearings (depending on the type of wheel bearing - See EDIT at bottom)
- Steering rack
- Springs (thanks for the add, GdD!)
Also, any time you disconnect any of these parts (eg: disconnecting ball joints to change out a drive axle), you should have your alignment checked, as it can be affected.
Things which shouldn't affect your alignment:
- Tire replacement
- Shock replacement
- Sway bar bushings & end links
Mind you, there may be others (ie: list is not all inclusive). While I've included tires in the "shouldn't" column, it's good to get an alignment done when replacing tires. This isn't because the new tires will cause alignment issues, but rather, a bad alignment can cause wear issues on your brand new tires. If you're going to put down serious money on a set of new tires, you want those tires to last as long as possible, so a good alignment is imperative.
EDIT: As a side note about wheel bearings. I would concur with @GdD (from comments) wheel bearings are not a "given" to need an alignment done. I would break that out as depending on the wheel bearing type. Vehicles which have wheel bearings/hubs all situated into one nifty little package probably wouldn't need an alignment. Other vehicles where you have to take the front hub apart to get to the bearing would most likely need it. This is because it never goes back together exactly the same. The alignment may not be far off, but could still stand to be checked.