fedup I think the answer to that is yes, and No.
I think most people discuss it, or at least think about it, at the beginning, when cohabiting is seriously discussed.
Then, once they've moved in, there is often a complete change in the dynamic of the family - a new baby, DCs become teens, or, sometimes, the DCs experience alienation due to an implacably hostile parent. So the goal posts have moved, and the DCs with whom the stepparent had initiated a positive relationship are hostile and/or unreasonable.
In my situation, I've set non-negotiables, after allegations were made and abuse thrown.
I will not share my home with DHs DCs all the while their mother is the dominating influence in their life. So, 50:50 or even regular overnight contact is no longer something I will accomodate. But, I accept that DH may choose that and am willing to live separately for a period of time if he believes that contact arrangement would be best for his DCs. I also acknowledge he may choose to end our marriage if I do that.
If their mum should be unwilling, or unable to care for them and DH becomes primary carer, then I will support DH to provide them with a home only with professional, therapeutic, support.