  Limb-darkening is fundamental in determining transit lightcurve shapes, and
is typically modeled by a variety of laws that parametrize the intensity
profile of the star that is being transited. Confronted with a transit
lightcurve, some authors fix the parameters of these laws, the so-called
limb-darkening coefficients (LDCs), while others prefer to let them float in
the lightcurve fitting procedure. Which of these is the best strategy, however,
is still unclear, as well as how and by how much each of these can bias the
retrieved transit parameters. In this work we attempt to clarify those points
by first re-calculating these LDCs, comparing them to measured values from
Kepler transit lightcurves using an algorithm that takes into account
uncertainties in both the geometry of the transit and the parameters of the
stellar host. We show there are significant departures from predicted model
values, suggesting that our understanding of limb-darkening still needs to
improve. Then, we show through simulations that if one uses the quadratic
limb-darkening law to parametrize limb-darkening, fixing and fitting the LDCs
can lead to significant biases -up to $\sim 3\%$ and $\sim 1\%$ in $R_p/R_*$,
respectively-, which are important for several confirmed and candidate
exoplanets. We conclude that, in this case, the best approach is to let the
LDCs be free in the fitting procedure. Strategies to avoid biases in data from
present and future missions involving high precision measurements of transit
parameters are described.
