@ Kristen - In a RACK or SSC BDSM relationship, the submissive always has the ultimate power - they just have to say 'no' and it's over. In reality, in all lifestyle relationships each party has equal power... though there are occassions when people let the fantasy of being mistress or slave go to their heads and they forget that no matter how 'real' it is, society and our laws say otherwise.
Topping from the bottom, in this instance at least, is simply meant to refer to the fragility of some dominants in their power in the relationship. If they submissive suggests or asks, or even demands, the top to do something, they get labeled as topping from the bottom and the dominant typically terminates the session and/or the relationship.
What you are referring to IS something that is important when you are playing with someone. The bottom does need to be able to communicate when they are experiencing levels of stress and/or pain that are in excess of what they can take, or when they are experiencing real physical and emotional pain (such as a pinched nerve, or going to a place mentally that is very bad for them). That's not really topping from the bottom - it's simply alerting your dominant that you are in distress and need to change or stop what you are doing. It's shocking to me (and I've been around this for a couple of decades now) how many people STILL don't get that, or worse, they get upset when they have to stop.
And I think you are absolutely right - once the top has earned the bottom's trust with their skills and respect of the bottom, it IS a much more rewarding experience when as the bottom you can let go and not have to worry about keeping an eye on your top. It's a very good opinion to have.. in my opinion!