What would happen when you under Bias your tubes?

Luna, the bias is supposed to be set so that the tube is being used in the middle of its operating range, in a region that is the most nearly linear for input-output tracking. Bias it too low or too high and it becomes increasing non-linear at either end of the operating range, respectively.

The actual value of bias depends on the type of tube. If you can refer to a tube manual, that may tell you the recommended bias. Or you can ask someone here who has a tube manual, listing the tubes you wish to have looked-up.

follow the manufacturer’s direction, try to stay within 10% low or high, 5% even better

The bias voltage is set to have a class A amplifier operating in the linear portion of the range.  The amplification across the frequency range will only be linear over s relatively small portion of the tube's amplification range.  You can easily go out of class A operation and introduce non-linearity to your signal thus providing sound that was not in the original audio source.

(Old Navy Engineer from vacuum tube days)

 I mainly set the bias to 70%. Is that OK?

Yes ALWAYS under bias. 
= Better tube life, less risk at blowing a  power tube. 
70 is OK

I mainly set the bias to 70%. Is that OK?

no

not if you want proper sound the amp maker intended