Too many times I've forgotten my own advice and gone straight to R&D, then when I remember to go to the VAB to assemble a launch, I didn't save enough for the hardware. I realize this wouldn't be trivial to implement, but it would be a nice feature if when you hit the RESEARCH PROGRAM FOR x MB button, the game would check to see if that would exceed the cost of assembling the missions you currently have in the VAB (as they are now, not taking into account the possibility of lowering costs by using, say, a B/Titan instead of a Saturn).

Or, perhaps, opening R&D/Purchasing would pop up a warning if you haven't assembled your hardware yet.