When weaving weft-faced circles I, like Claudia over at Mirrix Looms, always draw my circle right on the warp and follow it precisely. This means covering up the drawing completely. I’m not super picky about my circles being exactly symmetrical so my stepping will not be the exact same on all sides, but I do want to point out a couple of things about stepping (decreasing/increasing) on a circle that might not seem obvious to a beginner tapestry weaver, as well as some other maybe not so obvious tips.
1) Unless you are making tiny circles you do not need to decrease until you run out of warp threads. If you operate under this assumption you may end up with points. This entirely depends on your ends-per-inch (epi) and the size of your circle, but if you’re weaving three-inch circles over 6 ends-per-inch the top and bottom of your circle might be something like eight ends each.
2) Similarly my circles above have sides which have no stepping at all – they go straight up for 11 wefts (or picks), mirroring the top and bottom.
3) For weft-faced weaving consistent beating and tension are key! If you have even and tight tension you will be more successful at beating well and evenly and therefore your shapes will be keep their shape and stay proportional to each other. If you're weave two circles with different beating habits they will not have the same amount of picks and therefore may change shape as the tapestry progresses and/or after it comes off the loom. Remember that you should not be able to see your warp through the weft (This rule applies to every pick below the last one you wove; the last weft that was passed will sometimes show warp even though it has been beat hard enough.) In general you want to make sure you are beating hard enough to cover your warp, and then be consistent. But, you never want to be beating so hard that it becomes a workout. If If you are not able to cover your warp easily it's likely that you are using weft that is too thick, or conversely have your EPI set too close together for the weft you are wanting to use.
4) If you're like me and you enjoy the kilim-esque look of stepping and slits you needn’t read on, but if you require a nicely rounded circle you could try outlining your circle using the sumac stitch. I recently asked Kathe Todd Hooker (who writes these tapestry weaving books) and she put it like this
TIP: When drawing on your warp use ONLY fabric markers or sharpies you have tested for wash-fastness. Test them by drawing on fabric and then washing it to see if it stays. If the mark stays you are good to go. You do not want to risk your marker seeping through your weft when you block it or on a humid day.