As a general rule, never leave cells in the charger. IN particular, never leave cells in that charger, which doesn't seem to actually terminate (though I can't quite tell from that review).
As a general rule, ok to leave Li Ion cells in the light. Really, at this point your decisions aren't about safety (safety *is* an issue above, in a cheap non-terminating charger), but about cell life. Your cells don't want to sit around at 100% for months at a time, that will take some months or years off their useful life. So if I know I use the light often, like every week or two, this is easy: just use the light, and every few weeks or months (or whenever the cells start dropping to, say, 3.8V), top them back up. But if I knew I rarely used the light -- like only once every few months or less -- I might leave the cells charged to only 4.0V, which is an ok compromise between cell life and having decent charge if I *do* need it. That's all assuming there's not some constant parasitic drain.
Remember here that, in keeping your cells fully charged, what you're sacrificing is cell life. If it's not the end of the world to you to replace the cells in a couple of years, maybe it's a reasonable tradeoff of cell life vs always having a full charge in your light just-in-case.
You are, BTW, much better off asking this in the battery forum!