I'm only discussing bank robberies, but most modern bank robberies are completely unarmed, with just a note. Sometimes the note will say "Give me the money or someone will get shot" but mostly they say things like "this is a robbery. All 10's and 20s. No dye packs" or "Give me $$ or else." Usually they feature terrible spelling.
At least this is how it is in the Northeast, where most bank robberies are committed by opiate addicts. It may be different in other parts of the country where I'd assume the robberies are fueled more by meth. I will also acknowlege that bank robberies committed by non-drug addicts are more likely to involve weapons. Weapon bank robberies also seem to get more money. Note robberies is usually only in the $500-$2000 range.
@Zow can you confirm?