Hi Marte, Congrats on your new flooring! :) For our remodel, I also replaced every bit of carpeting and installed hardwood flooring in all the interior spaces -- kitchen, dining area, living room, stairs, bedrooms. (Tiles for all the bathrooms.) A general "guideline" I heard from the flooring pros is they usually install the floor boards perpendicular to the direction of the joists underneath. Of course, if you're gluing engineered hardwood directly onto concrete, that's another story. Also, if you have stairs that turn, then obviously all the floorboards cannot be all parallel, and some must be different. So the general guideline is just a guideline, not a must.
Are you installing solid hardwood or engineered hardwood ?
Do you have any photos to show your current (and planned) flooring ? That could be helpful.

Some people do and some people don't. If you are switching direction - place a horizontal threshold between the two. The example below illustrates the point