It is all about style and what you want to look like. However, do learn the most efficient possible way, then adjust it to whatever you wish.
I have an afrocuban stance, that means that I am shorter when I dance then when I stand up straight. That position is pre hustle, which is meant to enhance balance and body control for all that wild stuff rumba dancers do. The more afrocuban a song gets the lower I get to the floor, the more modern, the higher up I come…
As far as firmness, there should be no tenseness throughout the body, rather equal balance through opposite muscle groups. Meaning that your biceps should be as “relaxed” as your triceps creating balance…. Back as relaxed as abs, etc. The firmness though, I guess is best described as a skeletal firmness, where the skeleton is the on holding you up rather than the muscles. Not sure if that makes sense to you… tenseness is only applied when you need to act, so throughout the dance your muscles should always be as relaxed as they can be, but when you need them, bang, equal pressure starting from the core. Think of Bruce lee, the fastest possible way to move is by being relaxed, if you tense, muscles will fight one another instead of helping each other.
Stance, well, I think the best way to find out is to take a quick intro to ballroom. That intial stance is where all others originate, as it is really not a ballroom originating stance, but the body's natural stance... quickly, lean forward without bending the knees putting weight on the ball of your feet. Keep shoulders back but without applying too much pressure that you lose that relaxed state, head straight forward... from that you may adjust as you wish. It truly is as if you were about to take a step from a standing position with your shoulders in place and breathing through the stomach so that you don't have to worry about where your core is...