Mark,
You seem to be having problems fairly often on this website. I seem to remember helping you with it a couple times before, and it's usually a positioning thing. Is it possible that you've used negative values elsewhere and it has caused the very problems you're asking about?
Chris makes a very valid point that if you aren't fully aware of how other elements will react, you need to reconsider using negative values. I'll take it a step further: negative values should never be reactionary. In other words, if you're using it to push elements around in a very specific way for a well fore-thought purpose, that's fine. But, if you're using them to fix something that's broken, there's probably another mistake somewhere else.
"Why does it matter if it works?", you might say. I'd say once you're trying to add another element and it jacks it up, you'll find out. Something else somewhere is going to break. Then, you're troubleshooting that. And now your positioning is so convoluted that you can't troubleshoot it at all. You have to just continue the same broken system of pushing things around with offset values.