At render time all the faces get converted to tri's anyhow for rendering, if your model has all quads then the conversion is very predictable as it's a case of splitting the quads in half but if you have n-gons (5sided or more) then you have no way of knowing how the n-gons will be split to convet to tris and can give unpredictable results which is another reason to stick to quads even for a still shot.