Forum Moderators: open
Is it Ok or not?
You're unlikely to get banned for the odd hidden link here and there, but it's not OK in the sense that your page should be designed for people, and anything which is there (such as a hidden link) just for search engine crawlers is cheating. There are no two ways about this. You will maybe get better advice than I can give you about the likely technical effects on Google rankings, penalties, bans, etc, but cheating is cheating and the more it happens the more it's required for it to remain effective, and in the end the strategy (if that isn't too grand a word for it) will be self-defeating.