If you're playing a tourney, then I say anything goes if you're playing for money. If you're playing online, I say it's still valid online (there's no rule that anyone should hold back in any capacity) but do expect to annoy people.
In the end though, I think it kinda depends on the game. In SF4 it's understandable if it annoys people because it dominates the game so well for the most part. It helps to control space between the players and hopefully keeps you in an advantageous position in addition to building Super Meter. SF4 has fewer movement options and ways to counter fireballs than faster paced games with Super Jumps and Air Dashes.
I want to add a note though. There are certain ranges that constantly throwing fireballs breaks down in usefulness. The game does impose a hard limit of 1 fireball/player on the screen at once. A good player can take advantage of that to get a jump in on the opponent. I also favored characters with a special jump-in move that could defeat fireball users if I timed it properly (Sakura, Akuma, Dictator). Fireballs aren't fail-safe. People have to complain less and act more. Newbies are allowed to complain a little more, but veterans not so much.
In BlazBlue, we have air-dashing and barrier blocking. That makes it easier to punish a missed shot and block chipping respectively. I'm a Lambda player all the way, so I know that a good read or guess and air dash can destroy me. I also know how her Drive projectiles and specials let me have ungodly corner pressure and a decent mixup game, which is really quite exhilarating when I get it going. I do hold back if my opponent is a great deal more inexperienced than I am. It's not a real victory for me and it may put that player off from a good game if I destroy them. I'm not saying anyone else has to.
During my short month on playing vanilla MvC3, my team was Zero, Amaterasu, and Tronne/Wesker. I did run into the Sentinel/Magneto/Doom/Arthur teams who throw missiles/beams/more missiles/more beams. My team could deal with them because Ammy had an anti-projectile shield, Ammy could block out of her air-dash, Wesker had a projectile counter-attack, and Zero/Wesker had long-range dash/teleport moves. I could also spam back myself with Zero Sougenmu sword beams and Ammy Ice Beads. It turns out that those players have almost no experience fighting at close range and were quickly dispatched.
I understand that other teams/characters are much less prepared against heavy projectile users (Arthur's entire fighting style depends on projectiles and he has no business being in close range at any time. Anyone who says otherwise is deluding themselves). I still say it's valid and I actually enjoy learning how to eventually beat the next player who relies on projectiles so much. I still say that you should expect to annoy people.
Finally, on "cheapness." I dislike that term (go back to there not being a "hold back" rule anywhere). I find an enjoyable challenge in learning to beat players who rely on tactics commonly called "cheap." The only time I got significantly annoyed with a player was vanilla MvC3 where the opponent was Morrigan/Trish/someone else. They would super jump into a corner, lay traps, and rain fireballs for the entire match. They would dash away to the other corner every time I got uncomfortably close. I didn't have enough super meter to take them down before they switch/heal and had no other means of making them stay on the ground. I completely understand how effective that tactic was since I had no means to counter it and just want to express my personal annoyance with it. By no means am I telling that person to stop. Instead, I want to see them win a tournament with it. It will be boring as hell to watch, but I want to see them try.
