As a DM in a one PC game, I don't cheat. All dice are rolled in front of the player.
As a player, I don't cheat. 3e is complex enough in the rules and in the math that mistakes get made. But that's not cheating. Cheating is: "forgetting penalties, 'rolling' dice, failing to keep track of item/power usage..." I'd add the fast pickup of the die and serial advantageous "misremembering" of the rules to this category.
The worst effect of REPEATED player cheating is that it robs the other players of their moment in the spotlight. They never get the chance to be the unlikely hero. They never get the chance to be the one the party leans on. And it has an especially pernicious effect on players playing utility characters, like the ranger.
Moreover, it drains all heroism from the game when you remove the element of risk. The peaks mean little when there's no valleys and no sacrifice in the climb to the top.
And once a player gets caught after cheating frequently, I can't trust them again at the table. So it slows the game down to a crawl as the other players keep asking for the math. They may be fine people, but they're selfish at the gaming table.
If you can't handle the randomness of the dice, take the Luck domain, some Luck feats, or earn some action points. And if you can't handle failure even with those safety nets, well, go play a video game on God mode or go to therapy. It's not my job to enable your dysfunction.
Serial Player Cheating: It sidelines other players. It dulls the narrative's impact. It slows the game flow. It erodes party trust.
Don't do it. And if you did it last session, your penance should be to deliberately botch 2x the rolls next session at equally important times.