Borderless printing is not the only process that creates waste ink, nozzle cleaning does too. Even if you don't initiate a cleaning cycle, the printer driver will initiate one after a period of time, especially if the printer has not been used and switched on again.
I would not think the waste ink tank is actually monitored directly, I suspect the maintenance warning is based  on the total quantity of ink used, for reasons given below The waste ink tank will probably have sponge in it to avoid spills.

There is some Russian software used for re-setting Epson cartridges that also gives a readout, from the driver I assume, of the number of "ink shots" per colour over time - it's in the '1000s. The software allows you to zero these numbers to eliminate the maintenance message, not that I'd recommend doing it!
If you use Qimage and disable expansion for borderless, then over-spray will be minimised.

In that case, why use the borderless setting? In normal mode with 3mm page margins (say), make you print size 16mm smaller  then the paper size in both directions and use centre placement and you will still get your 8mm border.
Like most things with Qimage, there's more than one way of doing something: Using a 5mm image print border and Fit to Page will give the same result. This assumes the page margins are equal all round at 3mm.
NB. don't confuse page margins and image print borders, they are different!

Terry

Hi
The 3800 and 3880 maintenance tanks last a long time and don't cost very much, so compared to ink the cost is negligible. You also get plenty of warning when its about to run out, so get one in as it won't print when it thinks it's full.
On the 7600 and 9600 you could refill the tank with tissues (or other things) and reset the counter from the printer console. On the 7800 and later printers Epson closed this loophole and you had to buy a new tank, although I believe it is possible to reset the chip in the tank.

Brian