I think that the theory behind this statement is that dust only settles on upward facing horizontal surfaces, I cover my 10" DOB with a good quality plastic bag and then an old bed sheet after all it doesn't matter what it looks like when it's not in use.
Regards,
Greg.
A book I read lately said that I should always store the scope with its main optics facing down whether the dust caps are on or not.
Is it necessary to do this? 
[/QUOTE]