I transport kayak flat on fixed factory bars which have a slight curve across and are towards rear of hatchback. This cause two issues 1) if kayak not centered across crossbars then have tiny gap between rails-crossbar and 2) kayak sticks out the back if balance kayak front-back and tired of forgetting to duck then whack my head. So wondering how critical is “center” on crossbars (both across and front-back) and if weight forward is even better since maybe wind is a bit upward off front crossbar?
By weight forward I mean when parked the kayak sits on cross bar without tipping but if I lift gently on stern it will tip toward front.
The best way, IMO is to have the kayak balanced on the bars so that it is the most stable, and then use bow and stern lines to keep it stable front to back. They also keep you from whacking your head, because you have to go out and around the boat to not walk into the lines.
Centered? Not necessarily.
I can;t say I fully understand what you are explaining (can you post a photo?), but for most racks:
Centered left-right - not required at all
Not necessarily centered forward-back, but being balanced such that there isn’t a lot more weight hanging out the front than the back (or visa versa) is important. With most sea kayaks, tying just before and aft of the cockpit provides reasonable weight balance.
With longer boats on shorter cars, to get this balance pretty much requires that the boat sticks out off the back some. Most states just require a red flag if the extension is 4’ or more past the bumper.
It is best to center the boat on the rack not on the car.
Centering left to right is not at all necessary. As far as centering back to front, having the load points as close to the internal bulkheads as possible is preferred as these are the strongest points on the hull. Most people choose a side to make loading easier and often to carry more than one boat.
As Wayne_Smith mentioned using bow and stern line will help you to avoid hitting you head on the kayak. If you use an add on rack, they are also required by most manufactures for warranty purposes as well as protect your boat and the general public in the event of an accident or rack failure.
Flagging is recommended if your kayak overhangs by more than a foot or two. State requirements can be quite involved and vary state to state, but anything is usually enough to avoid a ticket.