Will i get banned for not using tank stance all the time in alliance raids?

[NaoSen]

No you cannot get banned, in fact depending on the way you are being told, chances are they are violating terms.

It is prohibited to force personal views or disregard the opinions of others. If a report has been filed and the prohibited activity is confirmed, a penalty will be issued.

It is prohibited to make statements such as:
"There's no way we can clear this with [suggestion]."
"Big pulls are normal here, so do it!"
"I don't care what you think, just follow my instructions."
"I'm not asking for your opinion."

They might be referring to the following but you'd have to be like intentionally shirking onto non tanks or such. If you are really paranoid about it just turn it off when you see yourself gaining aggro.

・Aiding the enemy / Uncooperative behavior / Lethargic behavior
Refers to an act of performing actions that give an advantage to an enemy (monsters, or the opposing team/players in PvP content) by not performing the necessary gameplay required of the situation. This may be combined with combat sabotage as well.
Key Points
Each player has a different level of skill, and in some cases, there may be a situation where the duty/content will fail. From the perspective of a skilled player in such a situation, a less skilled player may appear to be "adversarial/uncooperative/apathetic," but even if this is the case, it is not a violation as long as the player is playing appropriately.

For example, the following types of situations fall under the act of giving an advantage to the enemy (or the opposing team/players in PvP content):
"I don't want to heal because there is a player I don't like in the group."
"I don't think we can clear this anyway, so I'll just get hit by the enemy attack and go AFK after I'm knocked out."
"I'm going to join the opposing team as a healer and do nothing so my friends on the other team can win the PvP content."
If a report has been filed and the prohibited activity is confirmed, a penalty will be issued.

Please note that Square Enix may conduct investigations and issue a penalty in its discretion even if a report has not been filed if the act has been confirmed publicly through online video, streaming services, or other means.