Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $12.72   3.712%   $13.1922   $13.19   $13.20   $13.20  
  Purchase #2 $55.70   8.519%   $60.4451   $60.45   $60.45   $60.40  
  Purchase #3 $172.92   9.801%   $189.8679   $189.87   $189.85   $189.90  
  Purchase #4 $242.27   13.277%   $274.4362   $274.44   $274.45   $274.40  
  Purchase #5 $27.68   9.773%   $30.3852   $30.39   $30.40   $30.40  
  Purchase #6 $151.29   13.361%   $171.5039   $171.50   $171.50   $171.50  
  Purchase #7 $87.29   12.149%   $97.8949   $97.89   $97.90   $97.90  
  7 purchase totals = $749.87       $837.7254   $837.73   $837.75   $837.70