Withholding metformin pre-contrast

What is your protocol for withholding metformin before pt receives IV contrast for CT, IVP, angio, heart cath?? Based on recent literature, we withhold it the day of exam and 48hr after, making sure urine output remains good (if outpt), or there is no change in BUN/creat post exam (if inpt), before restarting it. Have recently found that other labs withhold it for 48hr pre-exam, and then restart after exam. Just curious what other labs/depts are doing, and their rationale.
Also, do you use nonionic contrast exclusively, or is its use based on patient risk factors (diabetic, renal function, etc)?
Thanks for the input! --- Diana