For single-handing, ideally you want the helmsman to be able to adjust the mainsheet without leaving the helm.
So for a tiller, most any location in front of the helm can work OK, including a cabin top traveller, a traveller located behind the helm would be bad.
For a wheel, a traveller behind the helm or immediately in front can work well. With a wheel, I always thought the traveller setup on the cruising J-Boats showed the ideal singlehanded solution, one where money is no object.
That said as much as I may single-hand my boat, I find this one consideration a difficult reason to justify a traveller anywhere but on the cabintop. My first concern with a traveller in the cockpit is the fundamental danger posed by the mainsheet during a jibe. Make the mistake of a flying jib just once, and someone can be seriously hurt. Loss of seatting space and conflict with a bimini are additional questions.
As with most boating decisions, its all a question of what your priorities are...