Upon further investigation is seems like in most modern garbage collectors this isn't an issue.
See [here](http://stackoverflow.com/questions/449409/does-assigning-objects-to-null-in-java-impact-garbage-collection), and [here](http://stackoverflow.com/questions/20823527/is-it-necessary-to-set-obsolete-references-null-when-they-will-get-reassigned), and the "Explicit Nulling" section of [this](http://www.ibm.com/developerworks/java/library/j-jtp01274.html?S_TACT=105AGY82&S_CMP=MAVE) article.

All of these articles pertain to Java's Garbage Collector, but they should still be relevant for Haxe.