"This feathering is one of the major things I look at in a bow. It tells me how much the bowyer actually cared about his creation. It either says, "I take pride in my work", or "Eh, I really don't care"."
Me too. It's one of the first things I look at. I've actually seen some halfhazard work in this department by some respected, well-known bowyers... go figure.
It's a very good start that you're conscious of it and want to do well there. I knew from my first bow that it was an area that would easily reveal me, so I was very meticulous about it from the get-go. As a result, I feel confident that there isn't a 'dip' to be found where the dips blend into working limb of any bow I've made. Nowadays, navigating this area while shaping and tillering is second nature and I hardly think about it. If you care enough to do your best on each of your bows, it'll eventually be just as routine for you.
On bows where there is a handle piece(to include dips) glued on, or on overlays, I don't pretaper anything prior to glue-up. I just glue it up square, then cut it out and shape it. I finess the dips with #49 and #50 Nicholsons, files, scrapers, and finally sandpaper... but don't use the scraper or sandpaper any more here than I do anywhere else on the bow, probably less, since too much of their use can create a washboarding effect.
If you're having trouble, it may help to write on it, or wrap masking tape around it there as was mentioned. It may also help to work rasps, files, and scrapers from limbs toward handle... so you're not coming down off the handle/dips and digging a hole there in the limb. On the tip overlays, work them from limb out toward the tips.
With wooden bows, it may be harder to finess this area on flat-bellied bows than on radiused. A radiused surface just seems easier to blend into.