Following are a list of the most common exercises for strengthening the legs. I will discuss the value of squats, leg extensions, leg curls and calve raises. Other exercises are very valuable and should also be done as a variation utilizing the same methods and mechanics of training. Unless you are in a mode of rehabilitation, I suggest doing moderately intense leg training no more than twice per week, preferably every fifth or sixth day. If in rehabilitation, perform a variety of specific exercises using lighter weights with more repetitions more times per week.
Any variation can obviously be productive.
It is purely a matter of choice. I can hear some of you nonlifting types commenting already. Sad to say, I've heard other chiropractors make negative comments on the value of squatting. Most comments revolve around the knees or lower back being unusually stressed, and that squats therefore should be avoided above all other exercises. To that I say, take the blinders off, doctors.