How essential are the non-essentials?

Within Christianity there are many doctrines which are absolutely fundamental to the faith; the Virgin Birth, the Deity of Christ, Justification by Faith to name but three. Across denominational lines all believers will hold to these truths, they are essential doctrines. One cannot truly profess to be a Christian and deny such things. Yet there are also many matters which are of lesser importance such as the issues of head covering, our view on the millennium or the form of church government. Our salvation is not dependent on us having the correct view on these issues, so how then are we to approach them? Are we to be indifferent towards them and consider anything beyond the fundamental doctrines of the faith to be of no importance? How essential are the non-essentials?