A theological system ought to be the product of exegetical study of Scripture, not a preface to exegetical work. Hermeneutical principles are first observed in the Scriptures themselves, even in a cursory and casual reading. Those principles are then applied in actual study of the text in the exegetical process.
This important order of principles and process is one reason that it is a bit of a misnomer to refer to a “dispensational hermeneutic.” Dispensational thinkers claim that they (are at least attempting to) consistently apply a literal grammatical historical hermeneutic to the biblical text. In that hermeneutic approach, dispensational conclusions are just that—conclusions. If we claim to hold to a dispensational hermeneutic, then on the one hand we are asserting our lack of bias in consistently applying an objective hermeneutic, while on the other we are showing our bias by claiming a dispensational presupposition. One can’t have it both ways. Dispensationalists have struggled with this to some degree. Reformed theologians, on the other hand, have virtually dismissed this issue altogether, readily admitting that theology drives their hermeneutic.