It sort of depends what you mean by letting them win the game. I dont think its ever good to let an opponent win if you are evenly matched or relatively close to it. Its deceptive and dishonest, and if the other person finds out, they likely will feel negatively about it.

On the other hand, I did just let my 6 year old win a game of magic the gathering, but he cant comprehend the cards and all of the different mechanics, so I dumbed down my play to match his. He beat me pretending to be a 6 year old, essentially. I think that's fair, and he's simply not capable of understanding the bigger picture, but I'd still like to play with him and he's still learning the game as he goes.