I've heard all different types of ideas on this. Don't fold your arms, don't cross your fingers, don't clench your fists, don't play with your clothes, don't put your hands in your pocket... Blah blah blah...
The fact is if you spend the whole interview policing yourself and thinking about whether or not the way your sitting is "the wrong way" then your not going to be yourself and it will be apparent to the people interviewing you.
If you like to cross your fingers or put your hands in your pockets do it. Make yourself comfortable so that you can be yourself, this is what the interviewers are looking for.