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 $25.73   10.279%   $28.3748   $28.37   $28.35   $28.40  
  Purchase #2 $37.89   7.799%   $40.845   $40.85   $40.85   $40.80  
  Purchase #3 $102.27   4.834%   $107.2137   $107.21   $107.20   $107.20  
  Purchase #4 $140.56   7.439%   $151.0163   $151.02   $151.00   $151.00  
  Purchase #5 $145.73   6.327%   $154.9503   $154.95   $154.95   $155.00  
  Purchase #6 $63.92   9.757%   $70.1567   $70.16   $70.15   $70.20  
  Purchase #7 $170.88   9.435%   $187.0025   $187.00   $187.00   $187.00  
  7 purchase totals = $686.98       $739.5593   $739.56   $739.50   $739.60