Bumpsteer is all about the geometry. By definition (and I know this has been covered before), bumpsteer is the change in toe angle as a result of suspension movement. Specifically, within that "sweet zone" of static ride height +/- 1.5-2.0", which is where the suspension SHOULD live while on track or on the street.
Note the bit about CHANGE of toe angle. If you put the car on an alignment rack (or set up strings, or whatever you use to measure your toe angle), and you compress the front end 1" or lift the front end 1", the toe ANGLE should stay pretty much the same, and with all stock components, it does. Disclaimer: If you're a "hard parker" with 3" drop springs or some other such nonsense, you'll be at the outer edge of the "sweet spot" in the suspension range. But, since you'll be living on the bump-stops, who cares about bump-steer?
Assuming you're on stock components, and are lowered around 1.5" or so (probably 2" max from stock), the tie rods and control arm are sitting pretty much parallel. That means that as the suspension goes up and down, both the ball joint centerline and the tie-rod centerline move in and out relative to the chassis by pretty much the same amount. If anything, they migrate slightly towards toe-in, but we're talking hundreths or thousandths of an inch delta.
If you're at stock ride-height, the front control arms are roughly parallel to the earth at static ride height. If you lower the car, the inboard drops, but the ball joint, obviously, can't. Now you have dropped your roll center, but remember that the arms and tie-rods are still parallel. If you install taller ball joints to correct for the dropped roll center, that brings the arms back closer to parallel with the earth, and raises the roll center again. All good, right? Wrong. The outer end of the control arm has been dropped (taller ball joint), however the tie-rod stud remains the same length. Now the arms and tie-rods aren't parallel, and as the suspension moves up and down, they describe VERY different arcs, and that introduces a change in toe angle as a result of suspension motion, also called bump-steer. This is what a bump-steer kit is there to correct.
Bottom line is that you don't need it if you're using stock gear, but you DO need it if you're using taller studs. Be ready to spend some sigificant time dialing in the bump-steer kit, though. Take measurements at static, lifted an inch, and dropped an inch, and plot the difference in toe. Adjust the spacers on the bump-steer kit. Lather, rinse, repeat until you have as little change as possible. Last time I set up a Mustang with a bump-steer kit, it took around two hours on a rack, with air, proper tools, etc. Can it be done at home? Yes. But you need patience and attention to detail. This is not what I would consider a "bolt-on" deal like a billet rear lower control arm.