How Much Free Space is Needed For a Storage Hard Drive

This is NOT a question about how much free space to have on a "Startup," drive.

This question is for a drive that DOES NOT have OSX installed on it.

I routinely keep my storage drives no more than 90% full.
(I know this may be overkill, as this is what is recommended for startup drives.)

I just picked up a couple of 3tb Seagate Barracuda XT hard drives to store my HD video files.
If I kept 10% free that would be 300gb!

How much free space do I need to maintain on this drive?
10%, 300gb seems ridiculous.

Is there another guide to how full a storage drive can be?
Can I fill it to the rim?
Does the size of the largest stored video file matter?

Thanks Dave for the input.
It makes sense to me that a storage drive would not need much if any empty space.

I have these precious files that I need to protect, and just wanted to make sure that completely filling up the drive won't jeopardize the data.