[Otter74]

That's a pretty good rule of thumb (use 6 of the 8 on the cassette) for the inner and outer rings. You can shift the whole cassette on the middle ring. If you're on the small ring and small cog on the cassette, your rear derailleur won't be able to put enough tension in the chain, and you'll probably drop it, especially if you're already on the small cog and then downshift in front. On the big/big combo if the chain length is correct you'll have enough chain that it will stay in gear, but won't behave well.