I usually keep any foreign currency after trips in my travel "box", as it is very convenient to have a bit of cash already on hand at landing when traveling again to the same place. I usually have about 50-100 USD per currency, as I like to have "enough money to take a taxi to the airport" with me as a backup option, which has also many times been useful when card machines don't work or other similar situation happens (cash only shop or taxi). I also have a public transport travel card for the most common destinations I go to, next to the currency.

These days I go to a local money exchange company about once a year with all the currencies I think I won't need in the next year and exchange them all back, they have "ok" rates (for cash) and only takes one fee regardless of the number of currencies you exchange at the same time, so exchanging even small amounts gives me something. I usually ditch all coins already the way home to some donation box (I passionately hate carrying heavy coins) so have paper money only.

I used to just keep the foreign currencies for years, but after losing about 50 USD total to the India 1000 INR rupee note becoming invalid overnight and some Swedish SEK notes expiring, decided to exchange back about once a year. That was a good reminder that money doesn't hold it's value unless invested ;-) Also learned that you can "trust" UK and US cash more than many other currencies, with paper money "guaranteed" valid "forever" (inflation does lower the value, of course).

PS. When I went to the exchange place the first time, they were like "huh, how many currencies did you say you have?" and it triggered some questions about where the money came from due to some money laundering legislation... But later no issues with around 10 currencies at a time.