What have you guys been using Feathers on

I use my feathers to rank up heroes

I had the same problem until some day I thought "Who cares? It's just a game, do what you have the most fun with!"
Here's what I experienced in order of randomly upgrading anything which looked fine, ordered by importance:

1. Spend them for new 5*s first. Try to figure out the ones you need the most, looking at your current team. This has the highest influence on your units, so I'd advice doing that first.

2. Anyone has one main team probably. If one of the units listed for inheritance is strong enough to become one of your 4 (or 3) true heroes: support him with everything you have! I did both steps with Cecilia for my Horse Emblem and I do not regret it!

3. Super-merged units are only senseful if they are the core of the team, so the one most important unit which is stronger than all the others. I actually try to keep my team on the same merge-level (+2 currently). But I can only invest into Cecilia anyway because the other ones are naturally 5* or from GHB.

One personal point: wait with upgrading Roy. I also have a 5* merged Roy but he is so much left behind from Weapon Refinery that I annoy the support until they enhance the Binding Blade because it is the weakest legendary weapon now. Even the (better) magical pendant Naga has been increased but not Binding Blade!

And of course point 4:
I'm still stucked between Tier 19 and 20 because I can't always use my 4 most powerful heroes. So a team core of 3 heroes might be better...