Hi beakie,
the most important thing with Eagle (and any other PCB editor) is connecting components properly in the schematic.
Changing grid size you shoud be aware of changing inches and fractions thereof, e.g. 1/10, 1/20, 1/40 etc.
Changing grid size at odd numbers you might not have connected nets as you intended. Nets look like being connected, but when highlighting them they stop in the middle of nowhere.
When changing to "board" Eagle only knows nets between correctly connected pins! Missing one pin can ruin the work of eight to ten hours of schematic work not leading to the expected results, especially if there are lots of components to be connected.
My advise for the future: Use the ULP "NETLIST" to check your work.
Take the time and compare the Eagle connections with the intended ones to make sure the schematic is 100% alright.
Changing to "board" the air wires will never be wrong!
Creating a board with more than 80 ICs and the size of 420X260mm you'll know what I talked about.
Once again: electronics work require precision from beginning to the final board design.
Regards
Boncuk