In offset printing, small body copy or thin type is best to have less colors to “register” on the press. Remember that for offset, we are hanging a plate for each color made of tiny dots that needs to come back together in the same spot at a very fast speed. There have been many advances over the years that makes registration much easier, however very small type will look cleaner and be more legible if it is just 100% black.
Large black areas in offset printing will look weak if only set to be 100% black. Drummond’s “rich black” standard is 60% Cyan, 40% Magenta, 40% Yellow, 100% Black. This produces a neutral black color with a good ink coverage.
Registration or 100% of all colors is not a good choice. This creates too much ink coverage and makes the drying time longer making you job take longer to print. You should stay under 280 total ink coverage in all areas of print (60% Cyan, 40% Magenta, 40% Yellow, 100% Black: 60+40+40+100=240).
For Digital work it may be better to stay with 100% black only in all areas.