Bluetracker
Tracks Blizzard employees across various accounts.
When reviewing statistics to balance the game, the developers should focus on Rank 1 through Rank 10 - not on lower levels, and not on Legend
One argument the developers have advanced for the game currently being in a healthy state is the variety of decks being played, both across classes and across archetypes. There's only one problem with this claim: it appears to be based on data from across all different levels of play, which isn't useful in determining the actual state of the metagame.
It's easy to see why data from lower levels (sub-Rank 10) isn't helpful: players at that level are less skilled at deck-building and have incomplete collections, and the decks they build tend to be less viable.
Less obvious is why the data from Legend isn't relevant. But anyone who watches a professional player stream for any length of time will notice something significant: while top players will focus on hitting Legend as soon as possible, once they get there they spend a lot of time trying out new deck ideas to see if they're viable. The reason there is a lot of variety at the Legend level isn't because all those decks are viable, but because top players are trying out a whole bunch of stuff to try to find new decks that might work in competitive play.
It's only in the grind from Rank 10 to Legend (and from Rank 5 to Rank 1 in particular) where competent players will use their best decks. Consequently, it's the decks being used at those ranks that will be the most informative in determining the state of the metagame.
TL;DR: For balancing purposes, Rank 1 through Rank 10 is the best place to measure how the metagame is shaping up.
Iksar
We generally don't look at ranks, but more of a percentile of players. The amount of players in a given rank goes under a great deal of fluctuation depending on what day in the month you look at. Whenever we are interested in this information, we look at all groups and not just one. Usually all the groups are fairly similar with occasional outliers like Freeze Mage, Patron Warrior, or Miracle Rogue that vary a bunch at the different skill level groups.