If peek-a-portal, portal standing, portal bumping, portal edge glitch and (more recently) re-portaling is considered not legitimate; sure.
If using cheats that are part of the game (such as typing specific commands into the chat, mostly old games) is not legitimate; sure.
If looking up in walkthroughs/lets plays because you are utterly stuck is lot legitimate, sure.
But after beating the game for a first time, I cheat like a [insert racist/sexist remark of choice], mostly because I already know how to beat the game "properly", and all replay value is lost.