The only time it might appear rude is if it could be mistaken for a bribe. This is really a pretty rare occurrence, but just in case I'll warn you not to try and tip any police officers or customs officials
I did a training course with the St. John's Ambulance in the UK back in May last year. The new guidelines had already come into effect then. The most interesting thing I found was that now the rescue breaths are now optional. Apparently, chest compressions are not only good at keeping the blood flowing round the body; the air that is displaced and replaced from the lungs by the action of the chest compressions alone is enough to have new oxygen enter the blood stream.
Of course, the best solution is to have one person do chest compressions and another do rescue breaths at the same time, as some others have mentioned already. Then you can swap over when one person gets tired - 100 compressions every minute is hard work! (Especially if the ambulance takes a long time to arrive...)
Promising costs nothing, it's the delivering that kills you.