Here's my take.
First, if you are playing competitively, there really is no cheap.
You're there to win, by any means. This is how you should play in ranked matches.
It doesn't matter what tier you use, but always use your best 2 or 3 characters. For example, I'm best with Sub-Zero, Sindel and of course Kabal. If you play me in Ranked matches, you probably won't see me use very many other characters (maybe unmasked Sub). Occasionally if I've repetitively beaten someone with these characters, I'll switch to my own personal mid-level characters. But rarely ever will I play with my worst characters unless I've been absolutely slaughtering someone.
My only problem playing with people in Ranked matches that MAY be considered cheap, is when people only know one super-high damage or infinite combo and that is all they ever do. Infinites make the game no fun. You will never catch me doing an infinite and if I do use an infinite tactic I will not do more than 2 or 3 iterations. For example I like to trap people in the corner with Kung Lao and do: Throw, Spin, Crouching LP (for show), Spin, Hat throw.
If I play someone in a ranked online match and they use an infinite, I'll finish the match. If the second match, they pick the same character and start the same infinite, I will drop. This is the ONLY situation that will EVER cause me to drop. I'll lose 30 matches in a row to you if you play fairly without using infinites, but I will not play with someone who can't win without infinites.
Now, unranked matches or casual play is completely different.
First, the infinite thing is a little different. If the person you're playing has a good sense of humor, they won't mind getting stomped by an infinite ON OCCASION.
Second, rotate your characters. If you win take a step down your skill list to your next worse fighter. If I'm really smoking someone, I'll try to rotate and use every fighter. This gives them a chance to face my weaker players like Shang Tsung, Jax (I should be better with him), or Classic Sub-Zero.
That's about it. It's kind of like the golden rule.
Try to think about how you would react if someone was playing you the way you are playing them. If it would make the game "un-fun" for you, it would probably not be fun for them either. And that's what we're here for,...right?