I didn't know you could remind them, I will from now on. I'm new to the chess world and I'm not winning a lot, however winning on time seems hollow and empty in my opinion.
To flag or have mercy?

It is their business to keep track of the time just as it is yours to keep track of your own time. The clock is part of chess. I would not remind nor would I expect them to remind me, in fact I would be annoyed if someone brought up my time because I don't think that is their business.

If they have only 6 hours left and haven't moved yet, they're probably off line and they'll get your message the next day after they've already lost...

There is a check-box in Online Chess settings to avoid winning on time (does not work on tournament games).
I agree in otb chess thatthe opponent's clock is none of your business; I never expect to be reminded if I say forgot to press it.
But in online chess...I don't know.
For the record he went over time 6 hours later and I "won" both games.

I agree in otb chess thatthe opponent's clock is none of your business; I never expect to be reminded if I say forgot to press it.
But in online chess...I don't know.
For the record he went over time 6 hours later and I "won" both games.
Haha of course :p
Try warning them 24 hours in advance.

When a player accepts a game, that opponent automatically becomes responsible for all aspects of the game That means all the chess rules and all the time rules, etc. If he or she runs out of time or breaks any rules of the game, that players pays the penalty, whatever it is. I don't believe we are doing a chess player any good by not holding the player responsible. He or she will not become the best player possible. Playing good chess is a serious business as life is. We become better players and live better lives, if we play according to the rules and accept the penalties as well as the rewards.

I play mostly tournament and match games, so I don't have a choice in those--they flag automatically.
In other games, I rarely claim the win for at least a week. I'm here to play chess, not to gather statistics, so I get more out of it if we play to the end.

Chess.com has a setting which sends you an email reminder when you're low on time in an online game, so reminding your opponent seems redundant. Especially since if they're logging in here they're going to see the time left in the game anyway. If they don't or can't log in, they won't get your message.
My opponent in online chess had 6 hours left. So I sent him a reminder. Now it's 2 hours.
How many people remind the other player [mercy] or flag without telling the opponent that the time is nearly gone [no mercy].
Are you a merciful person? Or are you a cruel, barbaric merciless psychopathic monster?