To be useful, weight data should be reasonably accurate. The day to day variations in the rubber bag are bad enough without the scale getting into the act and adding its own two pounds. If you don't have a reasonably accurate scale, buy one. You don't need a super-expensive doctor's scale, but your scale should be accurate to a pound or so. Electronic scales that use a strain gauge instead of moving parts are cheap and plenty accurate; that's what I use.
Many scales read a little high or low all the time: what scientists call ``systematic error.'' As long as it's only a few pounds and remains the same, this is no problem as long as you always use the same scale. But if a scale reads high one day and low the next, it's yet another source of confusion: the last thing you need. Try stepping on a scale four or five times in succession. If it reads the same weight within a pound or so each time, it's fine. If the weight jumps all around, for example 170, 172, 168, 175 on successive weighings, junk it and get a better scale.
Keep in mind the variation among scales when you're traveling. If you're visiting a friend and happen to step on his scale and it says you've gained 10 pounds overnight, odds are it's the scale, not you.