Again, it depends on what you are using for paint. Most spray paints are lacquer or enamel based, and most non-modelling spray paints are usually much hotter than a model paint, as they are designed to be used on other things besides styrene plastic, and thus could leave your kit a pile of goo.
Even Tamiya Spray cans are are a fairly hot lacquer, and could do some damage to bare plastic.
So you should primer your kit. It will protect the plastic, smooth out most irregularities, and make your color coat look better.
I agree with all the above.. Tamiya paint/primer is (in my opinion) the best on the market.
One thing though...if you have never used spray cans on models...keep one thing in mind:
Paint in spray cans comes out at a very high volume very quickly.Keep your paint moving...or runs and drips will occur.For me...I usually use spray cans on car models...because the surface detail on most aircraft models(engraved panel lines,rivets,screen detail) can quickly become flooded with paint if I'm not careful enough.Start with thin coats..rather than one thick heavy coat.
This is probably more info than you care to know at this point...but I thought I would offer it anyway.Let us know if you have any more questions...were here to help!