grammaticality - Mixing contracted and uncontracted phrases in the same sentence

Is there anything wrong with mixing contracted with uncontracted phrases in the same sentence?

Examples:

I'm not sure it is possible.

("I'm" is contracted, but "it is" is not).

I am not sure it's possible.

("I am" is not contracted, but "it's" is).

I know that it is not grammatically incorrect. But is it not recommended? Or is there any other reason to not use it?

Answer

Is there anything wrong with mixing contracted with uncontracted phrases in the same sentence?

No, there isn't. You can freely write a word contracting it, and write another one without to contract it.
As reported by Mr. Shiny and New, sometimes a word is written without to contract it to put emphasize on it.