I've checked legal section but it does not say anything about this aspect and i saw a couple of apps that already have a disclaimer (Stackathlon, Saggregator):

API Site names and logos (stackoverflow.com, serverfault.com, superuser.com, stackapps.com, meta.stackoverflow.com) are © Stack Overflow Inc. This site is in no way affiliated with Stack Overflow Inc. except for the fact that we use their awesome API to provide you this service.

and

This site is not affiliated with Stack Exchange, Stack Overflow or any of their family of sites. The data used to produce this page is covered under a Creative Commons license.

Do you have any guidelines to follow?

I've checked legal section but it does not say anything specific of this aspect apart from:

Do feel free to explain that your product is built on the Stack Exchange platform so people understand your product.

I saw a couple of apps that already have a disclaimer (Stackathlon, Saggregator):

API Site names and logos (stackoverflow.com, serverfault.com, superuser.com, stackapps.com, meta.stackoverflow.com) are © Stack Overflow Inc. This site is in no way affiliated with Stack Overflow Inc. except for the fact that we use their awesome API to provide you this service.

and

This site is not affiliated with Stack Exchange, Stack Overflow or any of their family of sites. The data used to produce this page is covered under a Creative Commons license.

Do you have any guidelines to follow?