A too-tight cast-on can, at best, make your project look a little funny. At worst, you won't be able to put the garment on.
Socks with a too-tight cast on will not go on your feet, and this usually isn't something that blocking can fix. If you have trouble with this, you have two choices: you can cast on using a bigger needle, or use two needles held together, as in this video.
Binding off too tightly can have the same frustrating results. Use a bigger needle in your right hand to alleviate this problem.