Small little secrets that don't affect your partner are okay to keep from them. Like, let's say you accidentally walked into a wall on your way out of work, I don't think your partner would lose sleep over not knowing that happened.
On the other hand, if we are talking about kissing someone else, hiding money, or a something your partner doesn't approve of, its not a good idea. If they found out, they would lose trust in you, and if they don't find out, the guilt should be eating away at you.
Everyone is different though, and some people don't consider certain secrets detrimental, while others do. For the same of simplifying your life and not having unnecessary drama, it's probably best do minimize any and all secrets that would have that affect.