Anytime you can avoid a contingent offer the better. Contingent offers are not nearly as strong non-contingent offers and there is always the chance of getting bumped by other offers. If there is no other choice but to put in a contingent offer then it is perfectly acceptable provided the owners of the home you are purchasing are in agreement.
References
Courtesy of HomeOwners Advantage and link that to their entry in the wiki which is at HomeOwners Advantage
