I bought an order of 100 51K-ohm resistors (SMT, order-stuffer in one of my Mouser orders) for my projects to use as pullups/pulldowns ... Works fine. I think the general idea is that going well above ~100Kohm (especially >1 megaohm) makes the transient effects of RF and noise in the circuit/processor significant enough to possibly cause false positive readings on the line, so something in the <=100K ohm range seems reasonable.