There was a girl who posted a picture of her full cap on here awhile back and she had blended highlights that were done in 3-4 different shades and it looked absolutely amazing.

I will have to disagree with Mr. New York on the over 35 comment I just turned 38 and I will probably have highlights in my hair well into my 40's! Now I get a little crazy (cutting my old full caps down into mohawks and such) but even if you want to keep it more conservative you can do blended highlights instead of sunstreaks.

Grey hair can certainly add realism, but at the expense of adding age. It's kind of like the density argument. Low densities look incredibly realistic but they can also make it look like you are losing your hair.

I guess it all comes down to crafting your hair to fit not only your level of recession in your temples, density, color, etc., but also the overall look you wish to achieve. The important thing is that everything looks like it's a part of you and not something that you stuck on your head.