I just came across this article this morning. I do the same thing. Certain stamps for certain things. Bills always get flag stamps haha. Never thought of it as a type of collecting until I read this article - probably because I don't really "collect" new issues.
For the past few years I have been using specifically themed stamps as postage...musically themed stamps for musician friends, flower stamps in the Spring, John McCrae..the Canadian poet who wrote "In Flanders Fields" around Remembrance day...Sports figures for my sports fanatics, car themed stamps for those into automobiles...and many other themes. My favourite is using stamps issued in the year of the recipient's birth on birthday card mailings. Fortunately I can still acquire a lot of older issues to use as postage at the monthly bourse here in Ottawa.