How many of you gutter your underage for touching colors?
For me an pertaining to screen printed apparel, almost never. I have been asked on maybe two occasions in my 24-25+ years. When doing so, I assume that in these cases, the press is extremely out of whack or the printer is not familiar with printing wet on wet. Low tension and heavy ink deposits are trouble for these shops. We may have a reason somewhere, but I've never experienced a need for doing it.
In direct ceramic screen printing, (like on coffee mugs), you do try to print so that the colors do not overlap. Only reason being, is because they do not blend at all ( and are very think inks). When they dry, they are raised off of the mug. If you put so may screens/inks on top of each other, they build an unwanted thickness and once they become too stacked, they can knock off the print with the next screen. These inks are not permanent until fired. It's a very chalky ink and must be applied while hot or in a liquid stage. They are very opaque. Decal printing on coffee mugs is a different beast.