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 $7.65   8.236%   $8.2801   $8.28   $8.30   $8.30  
  Purchase #2 $29.78   5.781%   $31.5016   $31.50   $31.50   $31.50  
  Purchase #3 $138.45   4.550%   $144.7495   $144.75   $144.75   $144.70  
  Purchase #4 $161.49   3.177%   $166.6205   $166.62   $166.60   $166.60  
  Purchase #5 $182.63   9.578%   $200.1223   $200.12   $200.10   $200.10  
  Purchase #6 $227.62   6.747%   $242.9775   $242.98   $243.00   $243.00  
  Purchase #7 $19.94   4.157%   $20.7689   $20.77   $20.75   $20.80  
  7 purchase totals = $767.56       $815.0204   $815.02   $815.00   $815.00