4.75v is the minimum according to the ATX spec. I wouldn't want to see anything below 4.8 for the 5v. ATX spec also says the 12v is good down to 11.4v, but we don't agree, and neither does another reference from the ATX spec which says it needs to be within 5% of the nominal value, and we tend to fail and discard at 11.7v. I have seen plenty of systems have full accountable functional problems with less than 11.7v or less than 4.8v.
It's not a bad idea at all. It still won't always tell you something is wrong, because it might not become evident for a week, or a month, or three years, but it will at least tell you if there is PROBABLY something wrong with it out of the box, and that's certainly worth knowing.