Is there a downside to using splines? For instance I am doing a 45 degree layout in a room. To get the longest starting row I thought I would just start near the middle of the room (bisected diagonally) and finish one half, then add the spline and then do the other half.

Will the fact that I will have staples or cleats in opposing directions cause a problem when the floor wants to expand or contract?

Hardwood flooring, if subjected to expansion from elevated humidity, will tend to expand in the direction the tongue points to. Since you are installing diagonally, it really makes no difference which way it would want to go.
Although I am not a fan of reversing direction within the body of a floor (unless necessary due to layout), there is really nothing wrong with it. In your case (diagonal install), it is in fact, desirable to create the longest starting line possible which makes it necessary to use the spline.

I'm not trying to hijack the post but where does one get these splines or slip tongues?

Thanks,

Jeff

I found them at a local flooring supply store. I would suggest you bring a sample board with you to check the fit. I thought these profiles were all standard, but when I brought them home they don't quite fit right with the BR111 3/4" flooring I have. I will need to shave a bit off the width.

In a pinch, you can fabricate them using 1/4 inch fir plywood ripped to 1/2 inch width. (approx).
The mill that makes our 1/4 round, baseboards, thresholds etc. runs us off about 20000 ft at a time when they catch one of their men with apparently too much time on their hands and scrap stock laying around.
Charge me next to nothing for them.