V-brakes go out of alignment for a variety of reasons. They won't brake as well when out of alignment, will wear the pads unevenly, and will be more prone to squealing. It's worth fixing. Stuff to check (in order):
- Check that your wheels are true and you haven't broken any spokes. (Maybe the brake is still in alignment, but the wheel is wrong). You need a pretty straight wheel for your brakes to work properly.
- Check that your wheel is straight in the frame. It's possible to clamp your wheel in at an angle.
- Check if the brakes move back and forth easily on their posts. If they don't, unbolt them and grease the posts, then put them back on.
- Check that your brake pads haven't shifted or moved (happens sometimes if they're banged, or if they're not tightly screwed down). If they are at a funny angle, unbolt them and realign them over your braking track. They should not be hanging off the brake track, and should not be contacting your tire when you apply the brakes.
- Check and set proper alignment. There should be little adjustment screws on each V-brake. Screwing them in will slightly push the brake away from the wheel, and screwing them out will slightly push the brake towards the wheel. Keep playing with them until they're up and down rather than crooked.
- Check the cable adjuster. There is likely a little barrel adjuster somewhere along your brake line (on flat bar bikes it's usually right at the brake lever itself). Tightening this will give less cable play and make the pads sit closer to the wheel, loosening it will give more cable play and make the pads sit further from the wheel.
If you follow the above steps, you should be able to find and fix the problem.
