"Can I machine a workpiece without using a work stop?"
"The collet draws back as it grips the bore of the workpiece. Any variation in the gripping diameter will cause the part to pull back farther or less than the setup piece. If you are only machining the OD of the part, then a work stop is not necessary. Unfortunately, 99% of the time this is not the case. Usually you will be machining a face or shoulder.