While there is some flexibility in choosing your courses, you also have to careful not to risk being close to graduation and finding you are missing courses. There are courses that have pre-requisites (for example, taking Algebra before Trigonometry and Trigonometry before Calculus), and if you go out of sequence with these courses, the later courses may not be available at the time you need to take them, especially if you are attending college in the evenings when selections are limited.
Students that are hoping to graduate in the four-year period need to be very careful the way they schedule their classes. In addition if you drop a class or have to retake a class it can throw your schedule out of balance. This is why some students choose to take at least a couple of classes during the summer—to give them some "wiggle room" in case something happens. Unlike high school you are not going to "fail" the year if you fail a couple of courses, but you will have to retake the courses, and if you are on a scholarship or grant you will probably have to pay for the courses yourself.
Those students who really want to graduate on time may choose to take some courses during the summer in order to offset any emergencies that may occur. It is much better to schedule ahead and find you complete your coursework early rather than to try to hold to a tight schedule and endure frustration when you discover you will not graduate with the rest of your class. Most colleges only hold graduation ceremonies once a year, so if you don't graduate at that time, you may have to wait another year for the ceremony even if you complete the remainder of the requirements the next semester.