Q:

Does adding the 'piercing' tag to warhammers require further tweaks from the GM?

Realistically speaking, war hammers were used specifically for breaking through armor:

War hammers were developed as a consequence of the prevalence of surface-hardened steel surfacing of wrought iron armors of the late medieval battlefields during the fourteenth and fifteenth centuries.

In Dungeon World there is the Piercing weapon tag, it means specifically that:

n Piercing: It goes right through armor. When you deal damage with n piercing, you subtract n from the enemy’s armor for that attack.

However, neither Warhammer nor Mace have Piercing. They have no difference from swords and axes, actually:

Short Sword, Axe, Warhammer, Mace; close, 8 coins, 1 weight

As a DM, I want to add 1-2 Piercing to war hammers in my game. My concern is, there is only one weapon that actually has Piercing in the default equipment (a "dueling rapier", whatever it is). In terms of game balance, does adding Piercing tag to warhammers break anything, forcing GM to justify consequences accordingly (for instance, make warkahhers heavy and harder to wield), presumably adding more tags (like Awkward)?
By "balance" I mean that game should feel fair, and nobody steals the spotlight from any PC.
Sister question: Does adding the 'precise' tag to daggers break anything?

A:

No, it doesn't break anything
Dungeon World, like the other *World games, does not depend on careful design-time balancing of classes/items/player moves etc. Balance in DW is a dynamic act that is continuously performed by the DM, by adjusting the hardness of the DM moves. 
If some aspect of the game seems to be overshadowing others, all the DM has to do is choose harder moves when it misses. This raises the consequences, making it a tougher choice for the players to employ it.
Example: Plain Sword vs. Piercing Warhammer

— I swing my sword around and stab the dark knight in the abdomen.
  — Ok, that's your plain Hack&Slash, please roll+STR
  — There goes a … 4! … ☠️⚡️
  — Hmm, the dark knight easily deflects your blow with his shield and brings your guard down. You notice his mace coming at your head from the left, what do you do?

vs…

— I swing my warhammer around and gore the dark knight in the abdomen.
  — Ok, that's your plain Hack&Slash, please roll+STR
  — There goes a … 4! … ☠️⚡️
  — Hmm, the dark knight easily catches your blow with his shield and brings your guard down. He catches the opportunity to bring his mace down on your helmet while your warhammer is firmly lodged in his shield. The world spins and your blood seeps into your eyes. Take 6 damage. It seems he's going to easily continue whacking at your head while you're stuck. What do you do?

That's the GM move: Show a downside of their equipment. You have many more of those GM moves in your arsenal. 
Note that you don't have to decide on using such an approach up front. You can slowly increase the stakes if a piercing warhammer seems to steal too much of the spotlight, and stop when it doesn't do that any more. 
The balance of the game does not depend on equipment tags. It depends on how you respond to player moves.

