I buffer MISO coming back from 3.3V devices to:
a) prevent poorly behaved devices from interfering with MISO from other devices when not selected
b) prevent 5V from being applied to 3.3V devices - a good example is Programmers connected to ICSP pins.
c) prevent input diode protection on 3.3V devices from dragging down 5V signals.
My method is typically use cd74HC4050 to buffer 5V down to 3.3V, and some form of HC125 to enable 3.3V MISO onto 5V MISO line using the devices chip select. Put a pullup resistor on the Chip Select line so the buffer is not accidentally active due to a floating pin when the microcontroller is being reset. You can see that in U9 and U10 here, with the pullup on D10:
(the links open to much more viewable schematics)