It's usual to have the saddle level because that's the most comfortable. Many people, including me, slide forward on the saddle frequently and on purpose. It changes what muscles I am using somewhat and freshens the nip on my butt. Don't worry about it as long as it's not making you numb or otherwise uncomfortable. I think moving around on the saddle is a good thing.

That said, do a saddle position check: while pedaling normally, hands on hoods and elbows bent, briefly take your hands off the bars. If you slide forward on the saddle, it's too far forward. Move it back. You don't want to be always pushing back which is hard on the hands.