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 $35.54   13.114%   $40.2007   $40.20   $40.20   $40.20  
  Purchase #2 $28.63   8.672%   $31.1128   $31.11   $31.10   $31.10  
  Purchase #3 $206.35   10.698%   $228.4253   $228.43   $228.45   $228.40  
  Purchase #4 $6.26   10.575%   $6.922   $6.92   $6.90   $6.90  
  Purchase #5 $133.74   3.844%   $138.881   $138.88   $138.90   $138.90  
  Purchase #6 $110.62   7.817%   $119.2672   $119.27   $119.25   $119.30  
  Purchase #7 $87.78   13.140%   $99.3143   $99.31   $99.30   $99.30  
  7 purchase totals = $608.92       $664.1233   $664.12   $664.10   $664.10