I use a 25 watt soldering iron to burn the markings on my little turtles. It works fine on the beech wood that I use, but it's awkward because the tip is such a long way away from the handle - three and a half inches on my particular iron, so it's similar to attempting to draw whilst holding a pencil a long way back from the tip - fine control is difficult. Depends on the physical design of your particular iron though.
Having spent some hours on the carving, I get nervous when it comes to the wood-burning stage because the iron really isn't ideal, and with just one slip my efforts would have been in vain. I'm not really a carver though - I'm an electronics engineer who just dabbles in a little carving occasionally. Oh and if I let the tip cool off even a little then it barely makes a scorch mark on the beech, so I need the full 25 watts. Here's the result I get on the turtles:
