I personally find that using a transparent png with 300dpi and 1:1 works best. I only use tiff's and jpegs when i don't need a contour cut and then directly import into the rip. With a transparent png you can automatically do a contour cut and not have to go through the extra steps of making transparent and deleting white space and then doing a contour cut. Saves me alot time. If i missed understood something when i first learned how to do contour cuts with non png's then please correct me, but i found this the best way. Really don't see a color difference with the png's being rgb instead of cmyk. LXI/Flexi screws up my colors every time when i try using vector files for printing. So i have found the png method the most time saving.
Only problem i have is with the actual printer. When printing bidirectional it prints with a grainyness. I tried increasing the heater temp to 45 and same result, tried different profiles, same result. I ended up just setting it to print unidirectional with a temp of 45 and my prints come out almost perfect.
A lot of times the graininess you get on prints, from otherwise healthy printers, is caused by by the motor speed. Go into your printer settings in Flexi and override the motor speed to slow it down. That should allow you to print bi-directional without graininess. Flexi profiles usually default to the highest motor speed.