Rules of thumb for un-identified LEDs?

This thread made me wonder what values of LED voltage and current one could use for an un-dentified LED found in one's parts box?

Could one use about say roughly more-or-less approximately 2.5V and 20mA?

http://www.kpsec.freeuk.com/components/led.htm

scroll down, has a decent table

Thanks- I use that site quite often (and even post links here) but hadn't seen that table....

5 - 20ma is a good number, and Vfwd would be Vfwd at such current levels.

Typically, they are good under 100ma.

I agree with 5-20mA for most 5mm round LEDs, but I bought some high brightness 5mm whites that are 30-35mA. Then there are the" 5-chip" 10mm whites that are 100mA (20mA x 5 chips?).