WOW! I was totally expecting a completely different conversation in this thread... LOL
Something more along the lines of "How high can I get and still safely flash a ROM"... LOL
Well, anyways, here is my two cents on this subject. Take it for what it's worth...
As everyone has stated here, it is completely dependent on your phone and how it reacts. There are many different kernels that have ranges anywhere from low end 125MHz-250MHz and high end 800MHz-1.25GHz. Then, each of those ranges have ultra low volt, low volt, and medium volt. There is not way to know which one kernel is going to work without just flashing different kernels until you get one that works wonders.
For me, I stick with the second "gen" Chevy 1.1 LV kernel. His new GB kernels will run on Froyo and I flashed the 1.1 LV, 1.2 LV and 1.25 LV and each booted fine. However, after anywhere from 1 hour to 3 days after using them, my phone would start to act really funky and would slow to a crawl, auto close apps (not FC), and random reboot. I switched back to the "older" kernel and haven't had a problem since.
So, again, it all depends on your phone. If the kernel absolutely won't work, your phone will bootloop. But, that is not the only indicator that your phone doesn't like a kernel. Like I said above, sometimes it takes an hour or a few days for your phone to let you know that it doesn't like the kernel you are using.
Just find your one trusty kernel and always keep it on your SD card. That way when you want to try a new kernel, you always are guaranteed to have your proven one available, if needed.
Kratos