Is it necessary to traverse every letter in the tape of 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.