I usually wait 'til I have a working relationship w/ the doc before changing orders/reading their mind. I have one doc who updates cisplatin orders but doesn't carry over changes to the pre/post hydration schedule, so I'll routinely strike out mannitol/K/etc... based on previous orders, or his particular practice. Saves everyone time.
With my inpt docs, my changes fall into basic categories:
1) Hard stop, page the doc, leave in queue until clarified (major stuff, major confusion, suboptimal abx regimen that I need to discuss, new doc I haven't worked with, etc...)
2) Change it/do it, page the doc an FYI (ie "hey you continued pt's home flexeril dose, just found out they only take it at night PRN, changed it for you, call me if you had something else in mind, ie "hey talked to RD re: TPN, made recommended changes, will order for you, let me know if you wanted to discuss").
3) Change it, route it back to the doc for a cosignature (more of a CYA)
4) Change it, no physician contact (timing issues, pt took the dose PTA & doc defaulted it to a now dose so change to next day, adding more specific PRN instructions, rates, etc...)