Quizzes should be used as part of formative assessment if you use them at all. They are a fast and simple way to get some feedback about what lower level skills your students know. However, my recommendation is that if you are going to use a quiz, use one that marks itself.

http://thatquiz.org and http://assistments.org both offer a free quiz platform which provide feedback to the students. Feedback is a critical part of the learning process, and the sooner is offered to the students, the more effective it is. Why not offer it immediately after the students complete the quiz?

The feedback from Thatquiz is pretty basic, essentially is your answer right or wrong and then a list of the correct answers at the end. You can customize the options a bit with Thatquiz and its strength is the ability to create custom quizzes, and you can even create questions with some interactivity.

Assistments on the other hand allows you to create extremely high quality assessments which can actually be part of the learning process. They provide the opportunity to create a really useful formative assessment. Students can be working on a problem (in math) and ask for a hint on a problem, and based on the work the student has done to date, the hint changes. The backend for the teachers provides a lot of useful information, including whether or not a student has asked for a hint on any particular question.

Either way, if you are going to use an online quiz, make sure it provides immediate feedback to the students.