Nah, no reason. As long as they don't get out of proportion of my quads, that's all I really want.
If you do a proper strength training routine like Starting Strength or SL 5x5 and maintain good form, you shouldn't have to worry about anything growing out of proportion. These routines are designed around a good distribution of compound exercises with free weights that inherently make use of the body's musculature in a natural and balanced manner. For example, a properly performed squat uses the quads and hamstrings very evenly. If anything, isolation exercises (such as hamstring curls) are what lead to imbalances, as they let you hit specific muscle groups separately and, quite possibly, unevenly.
You emphasized "below parallel" on the squats - should I not put on more weight until I can 5x5 a "below parallel" weight?
You should not go up in weight on
any exercise until you can do it with
proper form. If you don't know the proper form for the exercises you are doing, take some time to learn it before you head back to the gym -
Starting Strength is a fantastic resource for this. If you are going to spend months training with barbells, it is well worth spending $30 and a few hours reading the book to make sure you are doing it right.
The proper form for a squat is to go "below parallel", where "parallel" is defined as having the hip joint below the knee joint when seen from the side.
This diagram and
this diagram show the proper bottom position of a squat. Exactly how far below parallel you go depends on your goals, flexibility, and the type of squat you are doing. For example, olympic weightlifters will usually want to go as deep as possible ("ATG") as snatches and cleans benefit from very deep squats (you don't have to pull the weight as high). Not everyone needs to go that deep, but getting below parallel is the
bare minimum. This depth ensures that the greatest number of muscles are engaged, which maximizes strength development. It also ensures that the muscles are used in a balanced manner: only when you get below parallel do the forces from your anterior chain (primarily the quads) get balanced with the forces from the posterior chain (primarily the hamstrings, glutes, adductors, etc). This lets your muscles develop evenly and keeps your knee safe. If you aren't getting below parallel on your squats, you are doing it WRONG and need to fix your form before you go up in weight.