There are a few folks who do, but you're right - most test on the ear.
If it works for you, the only caution is to make sure the area poked is clean first, and has completely stopped bleeding before releaseing the cat after testing (to reduce contaimination possibilities)
And if you do want to try the ear again, know that you can directly target the small vein. If so, be prepared to test quickly and blot firmly to reduce bruising ... and to reduce blood flicks all over you!