You can't 'win' against another competitor in this game, so I prefer to think about 'cheats' as shortcuts like someone above has said. I play a lot of challenges though and don't use 'cheats' if they're not allowed. With one exception. I love starting out the majority of my gameplay R2R style, so I will 'cheat' to take away all their funds after moving a Sim to a lot.
During gameplay the only 'cheat' I'll use is to drag dishes to the trash/sink/dishwasher for Sims, but only in an attempt to save time if they're late for work or tired. Sometimes I'll abuse the system when they're just starting out to get more simoleons for my Sims such as; planting a cowberry, then immediately selling it in buy mode for a bigger profit, leveling up a Sim's vampire skill so they can turn low value fish/frogs into plasma packs, making my Sims drop trash when taking it to the bin so they can scavenge over and over for upgrade parts or set down just a roof piece for shelter from the weather.
I've stopped doing that stuff though for the most part because it just makes it too easy for R2R Sims to advance too quickly.
In build mode I use 'cheats' all the time such as; MOO, Debug and the loooong cheat code to turn off entitlements for career reward objects. When building for a contest/challenge I always have to 'cheat' simoleons for a Sim so they can playtest the lot for me and set the lighting. I wish we could set lighting and intensity in build mode, but it's not available for some reason.
I also use the CAS full edit mode 'cheat' to modify townies, premades and sometimes when Sims age-up if they look hideous and need some tweaking. Unfortunately, that happens a lot. o.O lol The only time I've ever used a 'cheat' for skills was to level up a Sim's cooking skill so I could send them to a lot I was building and make baked goods on a cupcake machine I'd temporarily set down. I needed them for an outdoor wedding reception lot. Afterwards, I 'cheated' to reduce that Sim's skill level to where it was beforehand.
I've only used the relationship 'cheat' once as well. I needed my modeling Sim to be in love with someone and vice versa to set up a love scene for a particular round in a contest. I also had to 'cheat' to give her love interest a death buff so it would look like she killed him afterward.
So, yep, long story short, although I don't use mods (yet), I use 'cheats' to enhance my gameplay. 'Cheats' just makes the game even more enjoyable and fun to play than it already is for me! <3