I think there are several levels of mastering. steps. I stick with the most basic ones.
it's still very connected to the mixing process.. all in one.
messing too much is bad idea. as well as browsing through mastering presets and find which one fits the best!! lol
if people want to put tracks on soundcloud, no need to worry much imo. but if you plan to distribute if for free, self sell or send playable demos... then yes, spend time mastering. or invest a bit and send them to a pro.