In Johto, it is traditional to trade away Sentret when receiving gifts. This is because of the fact that for all but a select few, pokemon can not be given away, only traded. Even if the ball is handed over, a trade must take place for the pokemon to acknowledge its new trainer. Sentret are used for this purpose because they are so common and easy to catch. (Trades, after all, need not be exchanges of equals.) Their habit of standing on their tails may help them against pokemon assailants, but against superior human eyesight they only stick out like targets, so one usually must only throw a pokeball to capture them.
Because of this, Sentret have entered Johto culture as a symbol of reciprocity. Children at Christmas or other gift-giving holidays will typically buy their parents tiny Sentret figurines, which parents often collect to remind themselves of years gone by. Charities will often display rows or whole halls of Sentret-related merchandise; it is not required, but there are many who wish to give something in return, either when desperate or after turning their lives around. Idioms refer to them as well; the generous are called "Sentret lovers", while returning a gift is called "Taking back your Sentret."
As for the Sentret themselves, most of them sit in pokeballs or as household pets, or are even released into the wild after the trade is made. But in addition to the trade, it is customary to hold a "Sentret battle", where the two traded pokemon face off after a set period of time to make sure the pokemon exchanged for the Sentret is sufficiently cherished. Usually these battles are formalities, but sometimes, even against beloved and highly trained new pokemon, the Sentret even win.