They won't find out later unless you do something blatantly obvious, but anyways, it's not always, needed.
If you're family is accepting of homosexuals, (or the sexuality you are currently) then sure, tell them, why not?
However, if they aren't accepting of sexualities, then it'd be best not to tell them, after all they don't need to know anyways. (my parents are homophobic and they don't know, i've kept it secret for 6 years, if you need any tips on keeping it secret if they are homophobic, just ask me i got you covered)
Now, if you're not sure whether they are or not, test the waters a bit, maybe bring up a subject without exposing yourself like "mom (or whichever parent you're talking to) my friend is being bullied for being gay, do you have any advice i could give him to help him?" this will flush out a homophobe, they will express their animosity towards gays at the bringing up of such a subject, if not, they will give advice and show they care and their concern if they're not, but then again, that's just testing the waters once, you can think up other ways to test.