Collecting Banknotes By Denomination?

Great question @ColonialCoinsUK & thoughtful answer @NumisRob.

I think a lot of us currency collectors start out collecting our own nation by denomination. That's what I did -always saving my best note & keeping 1 2nd best (or using my 2nd best to buy a better note!) I kept $1 to $20 notes. Once that simple goal was achieved (in about a year's time), I started on signature combinations. That didn't take long either so than I had to have at least one replacement, one special serial number, etc. My denomination sets expanded to include $100 notes.

That was about 40-50 years ago. After I upgraded every note & started buying tougher ones I pretty much replaced my Bank of Canada sets & started to acquire the first & second series (1935/1937) which were out of reach for me as a kid. That project started 20-30 years ago & that was just my own nation's currency. I sold off my $100 & $50 notes for rarer lower denominations so that $25.00 would be my limit (one I never reached as the 1935 $25 will likely be out of reach). Collecting by denomination was how I started but I abandoned that approach after 2-5 years.

I began collecting WORLD currency in the era of social media. It was clear (to me) that a lot of IG users were just acquiring banknotes from their travels or exchange houses. My Canadian denomination limit was $50 and I realized that for World currency this denomination limit would have to disappear since there were a few examples of hyperinflation notes (with fancy serial #) I wanted & I was thrilled that world currency seemed so inexpensive (compared to what I had been collecting).

I suspect that a lot of collectors start out by acquiring denominations but then find certain design motifs, special serial numbers, commemoratives, fractionals, short prefixes or some other avenue that interests him or her. When it comes to collecting world there's almost too many possibilities. I have also heard of the One of Each Nation goals & those folks often go for the lowest denomination they can get.

My british notes are organised that way (treasury and white notes by signature and then all the 10s, all the £1 all the £5 together.)
USA small bills the same, denominations held together (except some obvious sets like Hawaii or the North Africa bills)

I wouldn't think to do it with world bills given some currencies have devalued so much they are 100,000 denominations etc.

I try to get a full set when I travel and then back fill sets by type. I'd have to be really into a country to get interested in signatures and variants. Only UK and US have their own folders, I have a fair number of notes from some other nations (France, Australia and Bulgaria...) but these things can get quickly out of hand.