We give an extensive description of the renormalization of the Higgs sector of the minimal supersymmetric model in sloops. sloops is an automatized code for the computation of one-loop processes in the MSSM. In this paper, the first in a series, we study in detail the nongauge invariance of some definitions of tan{beta}. We rely on a general nonlinear gauge-fixing constraint to make the gauge parameter dependence of different schemes for tan{beta} explicit at one loop. In so doing, we update, within these general gauges, an important Ward-Slavnov-Taylor identity on the mixing between the pseudoscalar Higgs, A{sup 0}, and Z{sup 0}. We then compare the tan{beta} scheme dependence of a few observables. We find that the best tan{beta} scheme is the one based on the decay A{sup 0}{yields}{tau}{sup +}{tau}{sup -} because of its gauge invariance, because it is unambiguously defined from a physical observable, and because it is numerically stable. The oft used DR scheme performs almost as well on the last count, but is usually defined from nongauge-invariant quantities in the Higgs sector. The use of the heavier scalar Higgs mass in lieu of tan{beta}, though related to a physical parameter, induces radiative corrections that are too large inmore » many instances, and is therefore not recommended.« less