It really depends on the form and size of your candle as well as what you are casting. You should always rely on you instincts and use common sense but here are a few guidelines to help you gain inelegance on the topic;
A long phrase can be used on a taper however remember that symbols help to shorten the need to spell each word or idea out.
Wick to base for magic involving banishment or removal. Anoint with complementary oil in same direction.
Base to wick for drawing magic such as protection. Again anoint with complement oil in the same direction.
Carvings can be done around the wick as well as down the side.
Carvings can also be done around the candles body. This often is utilized when substituting a regular taper in place of a knobbed candle. Knobbed candles incorporate knot magic with candle magic.
If the candle is a human figure (poppet) for healing you will want to place symbols over particular parts that need assistance. As well as anointing with healing oil.
For more information I suggest the following books;
I also suggest using consecrated carving tools specifically for candles;