Shortly after I wrote my long post, I thought, now hang on a minute, how about burning a CD, for example? Filling it to only 75% is wasteful, is it not? Then I realized what the answer was. Yes, it was wasteful, but... a burnt CD is not a dynamic system! Once burnt, the content is static. So you do not need any slack.

So I guess the real answer depends on your usage pattern. If you do not experience any problems as it is, consider your N900 more-or-less set up and do not expect any changes in a foreseeable future, then it really does not make much sense to free up any rootfs space at all. Do not try to fix what ain't broken.

FWIW, here is my real life example: I bought my N900 second-hand. It had 67 MB rootfs free. I upgraded to CSSU and suddenly I only had 40 MB. I did not like the change but thought, what the heck, I'm not gonna need much space anyway. Until I decided to upgrade again, to CSSU-T and the damn thing complained that I needed at least 48 MB. I resolved it by running the upgrade from xterm instead of HAM but decided to set 40 MB as my limit. Come to think of it, it is only 17.6%, a bit short of 25% I preached before