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 $228.48   13.430%   $259.1649   $259.16   $259.15   $259.20  
  Purchase #2 $59.41   8.752%   $64.6096   $64.61   $64.60   $64.60  
  Purchase #3 $85.52   7.832%   $92.2179   $92.22   $92.20   $92.20  
  Purchase #4 $24.76   10.611%   $27.3873   $27.39   $27.40   $27.40  
  Purchase #5 $157.40   5.761%   $166.4678   $166.47   $166.45   $166.50  
  Purchase #6 $146.88   7.872%   $158.4424   $158.44   $158.45   $158.40  
  Purchase #7 $70.47   9.444%   $77.1252   $77.13   $77.15   $77.10  
  7 purchase totals = $772.92       $845.4151   $845.42   $845.40   $845.40