I think it's very important that inexperienced hamster people take away the idea that they shouldn't breed pet shop hamsters as there are a million pitfalls in breeding two hamsters with unknown genetics - as well as the sad fact that the hamster breeding mills are already breeding far too many "pet quality" hamsters than there are lifelong homes for.
If you breed to show - and are, therefore, breeding to improve type/colour etc in your lines (as well as the all important temperament of your hamsters) then having the odd totally unrelated hamster to throw into the mix is desirable. Most show hamsters are fairly inbred (I've only got to look at my pedigrees and compare them with Gill and Teresa - we've all got related hamsters, some related a couple of times over!). As Gill also says, she'll be breeding to a hamster with a full history and she knows (as well as it's possible to) that he doesn't have any dodgy recessive genes so, even if Peaches does, it won't matter as two are needed to cause problems.
At the end of the day I wouldn't think using lots of petshop hamsters if you only breed a few litters is a brilliant idea - Tristar and the other breeders who regularly visit petshops for new hamster are extremely experienced and know what they are looking for - they also breed a LOT of litters and have outlets for those babies who aren't show quality in the end - whereas the small timers like those of us here don't do that.
Having said all of that I'm not (really!) planning on breeding Aero, my very own petshop baby. However, if she ends up being an unusual colour
and and she grows and shows really well then I'd be stupid not to, wouldn't I?