here's a good research paper on drag and lift. even though it's mainly on large trucks (tractor-trailers) it does mention streamlined cars:
Information Bridge: DOE Scientific and Technical Information - Sponsored by OSTI
in regards to this thread, it says:
"For instance, the creation of lift on
the vehicle (effective weight reduction) can unload the
tires and reduce rolling resistance, while creation of negative
lift or downforce can increase weight on wheels and
traction, thus increasing braking as well as handling in
wet/icy weather."
so lift is a trade-off. you can either get less rolling resistance with lift, or more control with less/negative lift. it all depends on your concerns and driving habits.