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 $57.57   12.219%   $64.6045   $64.60   $64.60   $64.60  
  Purchase #2 $192.52   3.720%   $199.6817   $199.68   $199.70   $199.70  
  Purchase #3 $93.89   8.407%   $101.7833   $101.78   $101.80   $101.80  
  Purchase #4 $152.97   10.764%   $169.4357   $169.44   $169.45   $169.40  
  Purchase #5 $159.90   6.631%   $170.503   $170.50   $170.50   $170.50  
  Purchase #6 $124.65   9.871%   $136.9542   $136.95   $136.95   $137.00  
  Purchase #7 $158.89   6.872%   $169.8089   $169.81   $169.80   $169.80  
  7 purchase totals = $940.39       $1,012.7713   $1,012.76   $1,012.80   $1,012.80