It is generally recommended to use the frosted slides for Comet Assay. The frosted area allows you to do "labeling/marking with pencil/pen". This is quite helpful when you have lots of samples to run and you need to label the slides.
I am totally agree with Mr. Das, that you should have the frosted slides which should be pre-coated with 700µl of 1% (w/v) normal-melting-point agarose followed by drying on hot plate at least one day before experiment.