That looks about right. They are usually the "VIL" and "VIH" ratings in the "DC Characteristics" section of the related datasheet. For the 18X (16F88) at 5V ...
VIL : TTL <= 0.8V, Schmitt <= 1.0V
VIH : TTL >= 2.0V, Schmitt >= 4.0V
One has to look at the Pinout Description Table to find what sort of input pins a chip has, and they can change depending on mode. For the 18X, Inputs 0 through 2 are TTL, but Inputs 6 and 7 are Schmitt.
That means that some signals may work well with I0 to I2 but not so well with I6 and I7. Conversely, an RC debounced switch would be better suited to I6 and I7.
Connecting PICAXE output to PICAXE Input isn't a problem regardless of gate type as "VOL" and "VOH", the output voltages are designed to suit both types of input gate.
A table showing the type of inputs for each PICAXE wouldn't go amiss in the manual as an appendix.