The behavior of your female usually indicates when she is in true estrus - that part of the heat cycle when the female will allow a male to breed her. Most females growl or snap at the male when they are not quite ready for breeding because they still may be in the proestrus phase of the cycle. The proestrus stage usually lasts from 4-9 days and includes signs of bleeding, vulvular swelling and increased urination but this stage can last up to 2-3 weeks in some animals. Heat cycles vary from dog to dog and most of the numbers written are averages so this needs to be taken into consideration when deciding the time for breeding. Other females will exhibit this behavior when they are frightened, even if they are in true estrus. It is best to keep a log if possible of when the heat cycle starts - when bleeding and swelling is first noticed, when those signs disappear, etc. and time interval between heats if you have not already done so. This may help with determining the length of her cycle. Having your veterinarian perform cytology during the heat cycle will also help determine the optimum time for breeding. Using progesterone and lutenizing hormone testing may allow an even closer estimation of the time of ovulation. Some females would rather be bred at home instead of being taken to the male - feeling more comfortable on familiar territory. Other females may feel more comfortable (if bringing her to the male) to bring her a week or so before the expected heat (reason for the log), boarding her at the male's home so they can see each other and allowing them to breed on "their" terms. Then there are those females that will not breed no matter what. If it is really important to have a litter from a particular female that will not breed no matter what is tried, there is always artificial insemination.
The most common mistake in breeding dogs is simply missing the time when they are receptive to the male. This can happen as early as 2 or 3 days into the estrus and can be as late as 21 days after first signs of bleeding and vulvar swelling. The best approach is to attempt breeding every other day from the first days of the heat period. The only problem with this is that it requires a lot of cooperation from the owner of the stud dog.