Gary,
I have always understood that the 2 biggest cogs should be avoided with the big ring and one on each end for the middle. I have seen a couple of different ideas on the small ring. One is to avoid the 2 smallest cogs, the other to only use the small ring with the few largest cogs. Physially, you can pretty much put together any combination. The combinations to be avoided just put unnecessary stress on the chain. There is no need to use the extremes since the same gear inch combinations are available with less extreme angles. I make it a practice to follow the above recommendations and use the small ring only with those four or so largest cogs, which is almost never around here.
Regards,
Raymond