Depends on your entire chart... Venus and Mars and Moon and Mercury should be counted also... Sun Sign is jsut a BIT of you, tho it is best by rule of thumb to stay in the same element or a compatible element: such as:
Air + Fire
Earth + Water
But, if most your chart is water and earth and your Sun Sign is Air then it is usually best to stay away from Fire...
It is really complicated -- if I saw the charts I would be able to advise you better. I will use me as an example ~~~~ (each of these is how you act in blending with your Sun Sign -- if your Venus is in Virgo and Your Sun is in Cancer then when to comes to relationships you act like a Virgo/Cancer blend with people.... if your Mercury is in Gemini and your Sun is in Scorpio then when it comes to your communications with others then you are a blend of Gemini/Scorpio...)
Sun = Aqaurius with Capricorn cusp (You)
Rising = Gemini (Outer you and the mask you wear)
Moon = Pisces (Inner Emotional You)
Venus = Pisces (Relationships and Love)
Mercury = Capricorn (Communication)
Mars = Capricorn (Passion)
Saturn = Leo (RIGHT THERE -- Saturn is all about Karma AND Karmic CRAP -- my ex was a Leo...)
Jupiter = Taurus (Luck and GOOD Karma)
Mid-Heaven = Pisces (Who you aspire to be and what you become more like as you age)
Chiron = Cancer (A strong pull on who you are and how you come across)
NOW - if you will notice - my chart is LOADED with Earth and Water == this means ACTUALLY if I don't pair with a fellow Aquarian (the best match in the entire zodiac is 2 Aquarius people) then it is best I stick with Earth Signs or Water Signs.
I find I get along with all the signs -- as each person is thier own unique blend, but for me to get into a relationship with a Fire sign is a BAD idea unless the majority of thier chart is complimentary of mine...
There is a LOT to take into acount here, and to be honest I am going to bed really soon. e-mail me - i will answer as soon as I get the chance---