The two paths you've given mean different things.
If the path starts ../ that means "go up one level"
If the path starts / that means "start at the root level"
So, from the file domain.com/content/articles/help/entry.htm, the following pairs are equivalent:
../access/reading.gif or /content/articles/access/reading.gif
key.htm or /content/articles/help/key.htm
../../../styles/main.css or /styles/main.css
In some cases, the two paths you've given will resolve to the same destination (ie, any time when you are within the first folder down), but they do have different meanings and can be used in different circumstances, whichever is appropriate to the site you're working on.