The very first spell that most people cast blows a candle out. If you have had a birthday cake with candles where people sang the song Happy Birthday to You and then you made a wish prior to blowing the candles out then you have cast a spell. The success of this spell as with any spell depends on how realistic the wish you make is. Once a person realizes the power behind the birthday wish and the actions leading up to it they can then utilize the ritual to its fullest potential. The group of people singing raises energy as well as the candles burning. The quick end to the candles flame seals the wish. This is why people are encouraged to blow the candles out quickly in one breath.
Keep in mind that each spell is different and that snuffing a flame out may be better than blowing a candle out depending on how the spell is constructed and what the spell is for. Some spells require you to burn a candle for a specific amount of time over several days. The candle should then be snuffed or pinched out as to not end the casting but allow you to pick up where you left off. Spells that require you to burn the candle completely can be snuffed or pinched if for safety reasons you need to put the flame out. You can then relight the candle to finish burning it completely. If the candle must be burned all at once be sure to choose a candle that is not too large.
All of the options for burning/extinguishing candles are valid depending on the specifics of the casting. Some ways are better than others for specific spells. Remember that just because a candle is burned all at once does not mean it is more effective or more powerful. There are a lot more factors that play into the success of any spell than simply one detail like how the candle is burned.