To which voltage rail should I connect pull-up resistors?
For unidirectional lines, connect the pull-up to match what the input requires. Just make sure that when the output is high impedance that it can tolerate the rail voltage because that is what it will experience.

If you're talking I2C busses where the signal goes both ways then you need to take additional measures because now you're doing bidirectional open collector voltage translation so you probably need to get an IC to do the job. There's probably a circuit that can do it too with clamp diodes and resistors but I've not given it much thought