It depends on what you intend to do with the RAM.
CS0 maps to the "program space" of the MCU so memories controlled with this chip-select are usually extensions to the internal flash and are accessed through PPAGE.
CS2 maps to "extra EEPROM" and external space so access would be via EPAGE or GPAGE
Obviously if you intend to place the RAM in those spaces then the loss of the chip selects would make life more complicated. If you plan to place the RAM as an extension to the internal RAM then CS3 is the obvious choice.
The chip-selects are enabled under software control so if you don't need them then don't use them.
Have a look at
AN2708 for an explanation on the low level operation of the interface.