Fursty Ferret this has been debated to death I'm sure but I don't think it really matters whether you practice or not beforehand. I know that the previous head of recruitment was dead against it - and personally I didn't - but I know of plenty of ex colleagues from Flybe who did and passed their sim assessment. Maybe it depends on the instructor on the day but there is no hard rule you can hang your hat on saying not to have a go at a Boeing sim or even a 747 sim beforehand.