To answer the "Upgrade or Not to Upgrade?" question with DNN you really have to know which DNN features your site depends on and test them yourself. Actually you need to test them three time - first on a localhost then on-line, before you upgrade and again after the upgrade.
My site www.AgingSafely.com relies on parent portals and the ability to easily export and create portals. I have 11 now and am creating several a week now. Since I started using DNN almost a year ago with DNN 4.3.5 I have been able to upgrade only twice. This was usually because the export portal process exported all of the portals attributes, but the create portal didn't understand the new attributes added in the current or previous. To the DNN teams credit they have usually fixed this within a month or two. Then it comes back in the next stability release where some new attributed was add to support new functionality and isn't understood by the create portal process.
My testing strategy is:
On week after a new release is out I create a clone of my production system on a localhost and upgrade. Test as much as I can. If I can't upgrade I do a full install and try the items that I know I depend on. The reason I wait a week is that there have been two release that didn't pass the "week test" before they were replaced. BTW portal are fun to test on a localhost ;=}
When I am satisfied that it works well on my localhost and there doesn't seem to be a large number of problems discussed in the DNN forum I tested it at an ISP. If that works I upgrade and test again.
I am currently stuck at DNN 4.5.3 because 4.5.4 broke this feature and it "won't be fixed until 4.6.0." I'm sure others depend on other features and have to test them at each release.
Usually the x.x.{3-7} releases are the most stable and the dot-zero release, with lots of new functionality and therefore opportunity for bugs has the most issues.
I am looking forward to 4.6.0 and hope that one of the core team exports and imports portals in time to fix the bugs in that area before it ships.