It's encouraged, and likely to have a 'reward'.
"And when you are greeted with a greeting, greet in return with what is better than it, or (at least) return it equally."
"....But when you enter houses, greet one another with a greeting from Allah, blessed and good...."
There are also hadiths on it:
In one Hadith a man asked the Prophet about which aspect of Islam was best. The Prophet replied:
"Feeding the hungry, and saying salaam to those you know and those you don't know." (Bukhari and Muslim)
"The person closest to Allah is the one who precedes others in greeting." (Abu Dawud)
on this forum or any forum
Well, online or offline, you're still interacting with humans, so it'd be nice to give salams whenever you can :).