No, you dont have to convert them to master clips.
Hit M on your keyboard to do a Match Frame, then go to Clip > Edit Subclip and edit the clip as you wish. You have to manually shorten or lengthen the clip on the Timeline when you are done editing the Subclip.
I do like your suggestion better then the one I have been using which is to: Edit Subclip/Convert to master clip. Because sometimes it takes up to 10 seconds to convert the subclip to a Master clip.
I programed a hot key in my "shuttle Pro" to "edit subclip" so I don't have to even touch the mouse.