It depends on what boost levels you are. I would strongly recommend to use at very least a narrowband AFR gauge hooked up to the OEM sensor if you are on budget. If you have 300 bucks or so to spend than go with a wideband to be safe. Usually the factory ECU is doing pig rich and so you should be safe until a certain point but in the medium or long run you are better off with at least a SAFC or MAP ECU or other fuel piggy backs.
Monitor your AFRs and head for around 11.4/11.5 AFR at WOT..
I am with Gerd on this one, you need a Wideband. After you figure out how rich/lean you are in certain places it is time to adjust the fuel. I reccomend a MapEcu to utilize the power with no maf (converts to speed density).
I have been running the mapecu and the Innovate Motorsports LC1 with XD1 guage for about 6 months. It is pretty easy to setup and use. If you ever need any help PM me. I am about to get out of the piggybacks, and get an AEM.