SPI/I2C chip, CS# pin pullip for I2C operations
This post has been bumped to the homepage by Community, but I'll leave another answer just for variety and for reference by others.

In my experience one should always follow the data sheet. If the data sheet indicates a pullup resistor should be used, there's probably a good reason for it.


  is the resistor on the CS# line mandatory for the chip to communicate through I2C


In theory, a direct connection is just like a very low resistance resistor (a very strong pull-up resistor). In many cases, a direct connection is fine. Unless you know the internals of the chip, however, there could be a reason to limit current to the port. That's why the datasheet might supply these suggestions. Following the datasheet is a good idea. Otherwise test, test, test.