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 $27.29   4.422%   $28.4968   $28.50   $28.50   $28.50  
  Purchase #2 $198.63   6.628%   $211.7952   $211.80   $211.80   $211.80  
  Purchase #3 $34.40   13.177%   $38.9329   $38.93   $38.95   $38.90  
  Purchase #4 $123.47   12.756%   $139.2198   $139.22   $139.20   $139.20  
  Purchase #5 $184.86   10.638%   $204.5254   $204.53   $204.55   $204.50  
  Purchase #6 $87.87   8.241%   $95.1114   $95.11   $95.10   $95.10  
  Purchase #7 $108.38   8.250%   $117.3214   $117.32   $117.30   $117.30  
  7 purchase totals = $764.90       $835.4029   $835.41   $835.40   $835.30