When the children were younger, whether I let them win, helped them, etc., or not was dependent upon a lot of things.
While I think losing is as important a skill to learn as winning is, I also was very aware of the fact that there is a 6 year age difference between my children, and the youngest was unfairly and unevenly matched against the oldest and adults. I was also aware that if you are always the loser, you learn to hate playing games, and I think the fellowship pluses of a game outweigh the negatives of throwing a game occasionally.
If we were playing something even the little one could play fairly well, I played it openly and honestly. If he lost, he lost.
This might be chutes and ladders, bingo, or yahtzee.
You don't really need "skill" to win those.
You may occasionally need a little math help, or reading help, but that's about it.
My daughter loved Scrabble.
I would usually just sit those games out and offer to help my youngest.
Giving him first chance to find words with the tiles, pointing out special opportunities on the board like adding ing to a word already there, etc., but sometimes he wanted to play when it was just the 2 of us, or just his Dad and he and I, and yes, I'd go for words at HIS vocabulary level instead of words at mine.
Whenever I was teaching a new game, I would alternate between letting a child win and losing, until the skill set was built up and they understood the game.
If it was bowling or minature golf or darts, I often didn't keep score.
With something like Monopoly, I considered it somewhat unfair that adults understood the advantages of doing things like buying up all 3 of one color, putting on houses and hotels, or buying Park Place instead of Baltic Avenue, so I either helped with advice, or I didn't take opportunities that came around.
With a game like Clue, a favorite at our house, I often knew the answer long before anyone else, and just didn't ever bother to ask it. I figure it was more important that the children keep playing, and figure out how to use the evidence and elminiation process, than it was for me to go, "Aha! I know who did it!"
I think balance is what really matters.