What type of bike is it, and what diameter rim/tyre are you using?
From memory, most road bikes use 23 inch rims, and the 20/28 measurement should technically mean that these tubes will fit anything from 20 - 28 inches in diameter??? Therefore you should be fine.
Please correct me if I'm wrong though - I'm only passing this info on second hand.
You should be fine then. Good luck!
Presumably they weigh a bit more if that is an issue and might take up more space in the toolbag. Most problems arise going the other way. If you blow an inner up to bigger size than it is designed for it just gets really thin and easy to pinch or puncture.
Most road bike use a 700C rim ( approx. 27 inches ) in diameter and they have a nominal rim diameter of 622 mm
Most mountain Bikes use a 26 inch rim although a new size of 27 inches is coming into use.
20/28 relates to the minimum and maximum cross sectional diameter of the tyre that the tube will fit. The cross section I mean is when you measure from side to side of the tyre when its inflated.
Read this page http://www.sheldonbrown.com/tire-sizing.html for a better explanation.
You could also read this page http://www.sheldonbrown.com/tires.html for a good explanation about tyres