Weighing your pigs weekly will allow you to catch changes that can indicate they are ill. Weightloss is very serious in guinea pigs. What you are looking for is not necessaryily total accuracy but you do want it to weigh consistently so you can see changes. One or two ounces may be normal fluctuations but 3 or 4 (or more) definitely indicate something is going on. See your vet if your pig is losing weight and hand feed.
Here are a few pics of scales:
Pinta's scale:
Swannie's scale (you can buy it on
Amazon at the moment)
And here's the scale Mel uses (note the bread pan on top). This scale was $25 at Target:
