If you look at the 40X1 pinout in PICAXE Manual 1 Serial Output is on Leg 7, look in the 16F886/16F887 datasheet and Leg 7 is RA5, bit 5 of PORTA.
To control that leg the code would be -<code><pre><font size=2 face='Courier'> ; Bit 76543210
POKE $05,%00000000 ; Serial Out = 0
POKE $05,%00100000 ; Serial Out = 1 </font></pre></code> DPG summed it up nicely - Poking SFR's can lead to incorrect operation and can even destroy hardware if done wrongly. Simulators rarely emulate the full nuances of poking internal SFR's ( not even Microchip's MPLAB ) and it really requires a good knowledge of the internal workings of the PICmicro and an understanding why something should work rather than just guessing at a solution which could work. In this case setting all the bits probably doesn't have any adverse effects, but it's always best to have proven that as best one can before trying it.
Full marks for logical deduction and getting stuck in. One gold star taken away for not knowing if any damage would be caused or not