This is a pretty common practice around here. So many of us started with multiple density shooting head systems (myself going back forty years ), one each for different weight rods. With as many as eight heads in each standard head system and more still if the user expanded to include 20/10 head systems, keeping all of those heads straight and in order in our wallets by line weight and density was paramount.
Many of the high density lines - especially Cortland’s Hi-D heads ( Dark Chocolate and Black ) were so close in color tone, that in less than perfect light ( think foggy, grey, dawn and dusk periods. ) while steelhead and salmon fishing on the coast and interior valley, you simply couldn’t tell a type IV from a VI by sight. Likewise, once looped fore and aft, LC-13 heads were pretty close in color to some Deep Water Express heads and to Type IV S.A. heads all being in the grey tones.
Cortland did us a favor back in the day, with some of their old factory looped heads, by color coding the thread used to form each loop. But most of us would remove those loops and install our own braided loop systems.
So to differentiate head weights and densities, we’d mark the sleeve of our hand made braided loops ( these sleeves are much longer than the little Cortland jobbers that you can buy over the counter with those heat shrink tubes ) that ran up the line with a series of dash and dots.
Some would use various color ink markers to color code by line weight, then just add the dot - dash coding for line densities. Some guys even used brightly colored acrylic paint over white primer on the rear of the head to mark the line, in cases where the lines were to dark in tone, to be able to distinguish an ink mark.
Today’s useless bit of trivia: That the color / to line weight thing, sort of mimics what Sage did later on with their fly rod sections ( or a least used to do ).
If you look inside of the female ferrule ( you might need a pen light to see it ) you’ll find a single fine line - colored stripe. That stripe indicates the line weight of the rod, that the female section was or is part of. Then given the color ( finish of the rod blank, wraps and guides ) and the length of the section they are holding, they can determine the rod model that the section came from. Pretty ingenious Huh?
I still mark every new line that I get in, even if they are not going to get pressed into immediate service, simply out of habit.
Someday I may have to do this. I believe that if a fellow is in the midst of collecting rods - reels - and lines faster than he / she can become familiar with what's what this would be almost a necessity. Or if you are using several spools on the same reel body this would be a must.
I've answered the 'what line is that' problem in my own way. Each of the reels are dedicated to a rod and I know what they are. The Spey reels which I went crazy on over the past few years number at 5, however there are 5 rods too so I know what goes where.
I don't tag the lines per say, but I do have a spreadsheet with rods, reels, and lines on it. Each time I make a new addition, or change lines I just update the sheet. Works for me. Yes, I am a tackle junkie.