Here's a nugget from the intro in the back of W&H's printed volume (so, not the Intro/Appendix volume, but the actual text volume):

A few hours spent in studying a series of the countless corrections which no one would think of accepting will shew the variety of instinct to be found among scribes, the frequent disagreement between their instincts and our own, and, above all, the conflicting effects of different instincts in the same passage. (W&H, 542)

So often we (or I should say "I", instead of transferring my guilt to others) study only the juicy variants, discounting variants that seem obviously wrong or misguided according to common text-critical guidelines. This is exceedingly easy to do with the NA27 apparatus as guide, which (rightly for a handbook edition) focuses on listing variants but doesn't really get you any further in associating variants with particular MSS or even with particular scribes. In other words, studying variants with NA27, you will get a very good sense of the variation units but you won't get any sense of the underlying MSS and variant types to which each are prone.

W&H's advice above runs counter to the presentation in NA27. They say study all of the variants, not just the juicy ones. They say to get a sense of "the variety of instinct among scribes", not just items meaningful to translation. Reuben Swanson's NT Manuscripts volumes are much more amenable to this approach; they let you study each MS as a whole so you can actually get a sense of the peculiarities of each MS and, to some degree, scribes.

Why is that important? Coming to such an understanding by working through several of the seemingly "little" issues clues you in to how to process the big, juicy, more "meaningful" variation units. Otherwise one will likely just think things like "the shorter reading is the best reading" or "the more difficult reading is more likely the correct reading" or even "the older witness/higher quality MS has the best reading". Balancing those sort of seemingly objective criteria might be good guidelines overall, but they're hardly 100% applicable rules; particularly when these guidelines conflict, as they often do. What about when the shortest reading is the easiest reading, but it's in the earlier/more reliable witness? Or when the longest reading is the hardest reading, but it is in an 8th-9th century MS that doesn't agree with the major uncials? In these sorts of cases, a "majority rules" approach has as much chance at being right as it does at being wrong.

So take W&H's advice sometime. Work through all the variants you can find for your passage, keeping track of source (which MS they come from) and nature of variant. Use that information when considering overall which readings you consider proper. Have fun!