Is it necessary to traverse every letter in the tape of a Turing Machine? Say I want to make a very simple Turing Machine that accepts only strings that contain one or more a's. Can I simply send have the machine move a HALT state once it reads one a, even if there are many letters after that first a? Or would this be bad practice or unacceptable for a Turing Machine?
To the title question - no, to the flipped version of the same question in the body - yes. It is typically fine to halt without processing the entire input (however see Raphael's answer for certain technicalities in this respect).

There's not really such a thing as "good" or "bad" practice with a Turing Machine in the sense that there are "good" and "bad" practices when programming on a "real" computer. 

As they are theoretical models for computations, they are at their best when they prove (or assist in proving) what you need to prove in as clear and direct manner as possible. In this light, as long as your machine only performs permissible actions, then everything is fine - so no cheating, but beyond that it's all good.