HI - First time post. I am going to be changing most of the flooring in our house, tile and hardwood. The sub floor is 3/4" plywood and 2x10 joists 16" oc. I installed 3/4" x 2 1/4" oak nail down hardwood in our den approx. 7 years ago and installed running parrallel to joists. Installed parallel for astetic reasons. Have never had an issue with flexing, cracking or seperating. The area is about 9 ft. x 10 ft. Now we want to add more hardwood and to keep existing would have to continue with parrallel installation. The den area would expand to about 12 ft. x 20 ft. plus hallway and front entrance. Also running parallel would keep the boards running the length of the hallway not across; again astetics. Do not really want to rip up existing hardwood to add more sub floor due to floor levels. Do you see any issues doing this being that we have not had any problems with existing hardwood floor?
Thanks a lot
George

Here's the deal George,

Many of us that reply to DIY questions are pros. We work everyday on hardwood floors. We are licensed and insured. We know what can be done and what cannot be done. We cannot tell you it is OK to install a floor that is a contradiction to what the manufacturer, NOFMA, the NWFA and other authorities say. In situations like this, we'll tell you "the rules" and say, it's your house and your floor, do as you wish. Now, if you can gain access to the floor joists below, adding blocking between the joists will satisfy all "requirements." If you don't want to, as I said, we can't give you our blessing but it is your home to do with as you wish.

Pleae keep in mind that that very small initial room is probably a low traffic area. Opening it up into a larger space, and doing high traffic hallways etc., is not the same situation.
Adding bridging to the joist system will help a lot.
Keep in mind that the floor is not going to collapse because you are parallel with the joists.
You may get some squeaking and movement of the floor, you may not.
So, aesthetically, which is more important to you? The possibility of some minor squeaks that may or may not develop, or having a floor that is oriented in the most visually pleasing direction?