The traditional source of heat used by pen repairmen since the 19th century has been the alcohol lamp. Many repairmen still swear by it, but there can be little question that it is now obsolete and should be avoided if at all possible.
There are two main problems with using an open flame for pen repair. The first is that one cannot control the temperature of the flame. The second is that the column of heat rising from the flame is prone to shift invisibly and unpredictably with the slightest room air currents. In practice, this means that one has little ability to heat a small area to a precise temperature, and one runs a considerable risk of overheating (sometimes to the point of combustion!) and of applying heat where it is not needed.
The best general heating method for pen repair is now the heat gun -- but not just any heat gun. It is essential to have one that has a temperature control that is infinitely adjustable, usually by means of a dial. Most of these come with a range of nozzles, the smallest of which can direct the heated air into a column no wider than the average pen barrel. Most also have a range of fan settings, and can be rested pointing upwards for hands-free operation.
Although some open-flame diehards maintain that there are things than can be done with an alcohol lamp that cannot be done with a modern heat gun, this is a patently ludicrous claim. Heat is heat, and a heat gun can put out however much you need, anything from 120°F to 1050° (50°C to 565°C), the top setting being equivalent to that of a blowtorch.
It is in delivering lower temperatures, of course, where the heat gun excels. In pen repair, one generally wants to let the heat sink in. Heat is applied to the outside, of course, but the goal is to warm what lies underneath. Using open flame is like trying to do all one's cooking with a broiler: it can be done, but it's hardly the easiest way of heating an object evenly all the way through -- especially if one is concerned about scorching its exterior.
Passing the object to be heated rapidly through the flame is the usual method of tempering the heat of the lamp. Nonetheless, it takes considerable experience to avoid damage and even ignition of the workpiece. Furthermore, even brief, intermittent exposure to the temperatures generated by an alcohol flame are enough to weaken or erase imprints and chasing and to cause puckering at seams -- not to mention more insidious effects, such as the subtle degradation and embrittlement of materials through volatilization of plasticizers. Holding the object to be heated high above the flame is another method of reducing the heat applied, but then one cannot direct the heat very precisely, nor can one be sure that there are not hot spots even well above the flame.
So why do so many old-time repairmen still use alcohol lamps, and why are they recommended in many official pen company repair manuals? The answer is inherent conservatism and lack of exposure to new and improved methods. In fact, properly controllable heat guns seem hard to find outside of the USA (some time ago we tried to find 220V units for friends in Europe, but could only find the far inferior versions with only two or three fixed heat settings), and while alcohol lamps cost almost nothing, a suitable heat gun will cost around $100-150 from sources such as Sears or Black & Decker.
ADDENDUM: Although the discussion above addresses the use of alcohol lamps (or alcohol burners, if you prefer), it applies only the more strongly to other forms of open flame. Alcohol burns at a comparatively low temperature, whereas candle flames are significantly hotter -- as well as less consistent. It should also be noted that several repairmen have reported good results with inexpensive heat guns sold at hobby and craft shops for embossing, despite their limited range of heat settings.