SPI and pullups. Is it ever used in open collector configuration
As far as I know, SPI lines are always actively driven in both directions.

I sometimes put a pulldown on the MISO line because it could be floating for extended periods of time.  Only the slave with slave select asserted is allowed to drive the line, so when no slave is selected, the line floats.

Floating MISO doesn't cause any data problems because it only floats when its value is meaningless and the master shouldn't be looking at the line anyway.  However, intermediate voltages on some digital inputs can cause trouble.  They can cause oscillations and higher than specified current draw inside the receiving chip.  A weak, even 100 k&Omega;, pulldown keeps the line at the logic low level when not in use.

I usually use a pulldown, but you can use a pullup just as well.  This only sets the line's data value when not in use, so you can pick either.  To me it seems more intuitive to have the line say "0" when not in use, but this is really totally arbitrary.  If you might fully power down some slaves when the SPI bus is not in use, then it should be a pulldown.