Is it important to keep components and traces on a PCB aligned to a grid?
Ultimately, the grid your design actually uses is built into the Gerber file you export from your PCB tool. The files that I generate from Eagle are in inches, with either 3 or 4 digits to the right of the decimal place. That puts me on a 1 mil grid. 

When you're using parts with different units (Metric vs. Inches), you're bound to have some grid mis-alignment, but it will be very small. If your design uses the minimum spacing rules, it's possible some spacing violations may come up.

In short, the only grid that really matters is the one you export your design to. Any other grid is for convenience.