the use of css which may not be supported in all browsers, what's a good rule of thumb on using it? When is a brower version sufficiently old or browser itself too insignificant to be coded for?
I believe coders have to decide that question for themselves.
One way is to cease explictly thinking about any user agent that has less than a certain % of the target user market. 5% was a common number for some time - but then I believe pages should pass colour contrast tests to accommodate user vision issues, and should provide for non-visual user agents (especially assistive devices) which are both usually smaller than 5% - so when I've applied that "rule of thumb" I've immediately ignored it in relation to certain user groups. ;)
Another approach is to code from the perspective of "progressive enhancement" or "graceful degradation". That also requires a decision about which browsers to cease explicitly supporting - a search will bring up lots of discussion about the issues involved with making that decision.
For me though, the important thing is to keep checking against your target user group: Alhough we code for browsers, the sole purpose is to deliver content to users.