It depends on which exact pins you want to use.
If you truly mean the i2c pins, then the best way to refer to them is as SDA and SCL.
That way it works for any board vs just an UNO.
So if you have a R3 or a leonardo, or mega that has the SDA and SCL header pins up by the USB connector,
the code will always use those pins.
However, if you want to refer to the header pins down towards the opposite corner of the board from the i2c pins, where the analog pins are, then use A4 and A5.
That way you will always get those two pins.
Remember that pin mappings are different for each board type so it is always best to use a symbol when available to ensure that you always get the physical pin you want. This is particularly important for the pins like i2c or the analog pins where the mapping to the digital pin numbers are quite different between boards.
Because of this, I would not recommend using the arduino digital pin number since depending on the type of arduino, (you may change it at some point), you may or may not get specific physical pins you want.
To use them, use the names as the pin in the API calls:
pinMode(A5, INPUT);
val = digitalRead(A5);
pinMode(SCL, INPUT);
val = digitalRead(SCL);
--- bill