Yes, depends on usage and underlying storage system. Some systems, like high end SAN based disk arrays laugh at file fragmentation making the only system impact of fragmentation is OS overhead in scattering things all over hither and yon. Other systems, like laptop, drives are another story all together. And that doesn't get into newer file systems, such as ZFS, where the concept of a hard limit to space is nebulous at best.
NTFS is its own beast, of course. These days I give C:\ a total size of 15GB for XP systems, and haven't played with Vista/Win7 enough to know what to recommend there. You really don't want to get much below a GB free on C:. Using Shadow Copies means you should keep more 'empty' space around than you otherwise would, and I'd say 20% free-space is the marker for when more needs to be added or a clean-up needs to happen.
For plain old NTFS data volumes, I get worried when it gets under 15%. However, if the drive is a 1TB drive, 15% is still a LOT of space to work with and allocate new files into (the converse being that it takes a lot longer to defrag).