Ok, define a little bit - a cup, a gallon?

And yes, you should change the coolant on a regular basis.  A flush is
overkill, but you should at least drain most of it and refill with fresh
coolant and clean water.

http://www.angelfire.com/ia2/vmax/coolantnotes.htm

has a bunch of good quotes (and some annoying popups) explaining how
coolant gets "used up" over time.  The coolant doesn't get used up, the
anti-corrosion inhibitors get used up.

And, if you own an old car that wasn't maintained properly before you
bought it, you'll usually flush all sorts of gunk out when you start
changing the coolant.

Story:  Friend bought a used 77 Dodge van for $1 back in about 92.
brown stuff in the leaking rad.  Swapped rad, flushed coolant.  Twice.
Two days later, had to do it again.  Took it to a shop a week later
because it was still brown.  They powerflushed it.  A week later they
did it again for free because it was still brown.  It took a total of 7
flushings before the coolant would stay green for more than a week.
(it was a slant-six, so it was pretty much indestructible - I think it
might still be on the road today...)

Ray

You're confusing me with the OP.  I wanted to know if he's adding a lot
of coolant to cover up a leak (2.8 intake gasket?) or if it's just
evaporation (the overflow bottle on my 86 Jimmy isn't exactly airtight...)

Ray