It is always advisable to clean threaded parts before applying the thread ring to gauge part. If the radial slots in the ring gage accumulate chips and dirt, it is likely the parts have not been cleaned and /or the ring gage is being applied too harshly and removing material. Remember that a ring gage is a precision gage and not a die. The ring gage should not be used to re size over sized and defective product. The ring gage should turn over the part with very little force or drag. Many books state that the customary amount of pressure is 2 pounds of force but there is no specification or standard that dictates this.
On the whole it is suggested to turn the part onto the ring but this should be determined by the configuration and size of the parts. If the workpiece is large and heavy then of course the ring should be turned onto the part.