Cheating is one of the biggest sins to most gamers.
It is cheating to make up rules as you go along unless one of the rules is "We make up the rules as we go along."
It is cheating to roll your dice behind a curtain.
Just doing something because it is your whim without regards to the rules is cheating.
In your game, you may have an assumed rule of "it is okay for the ST to cheat in my game" but in my games, it is never okay to cheat.
If you have a core book your game operates out of, abide by those rules to the best of your ability.
If you plan on having rules changes to those found in said book, make the rules publicly available with reasonable advance notice.
Example: if you plan on changing core books, six months notice.
Example: if you plan on adding a damage, or a retest, or a trait... one month or one game should suffice.
If you show up with new rules at a game, you can not play with those rules until the next game unless ALL of the players at the game in question agree on the rules.
Players should NEVER EVER have to "just know" things without referrence to something published somewhere. "the ST said..." without that having been accessibly published somewhere... is not a rule.
As an ST, it is your job to make sure that ALL abide by the same rules unless ALL involved in a matter decide on different rules.
No ammount of points spent justifies "I just win!" it is NEVER a power or background or ability or trait. There should always be some measure of WHY a character triumphs... especially when said character is an NPC.