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 $7.81   3.867%   $8.112   $8.11   $8.10   $8.10  
  Purchase #2 $129.67   7.849%   $139.8478   $139.85   $139.85   $139.80  
  Purchase #3 $10.79   12.863%   $12.1779   $12.18   $12.20   $12.20  
  Purchase #4 $74.82   10.652%   $82.7898   $82.79   $82.80   $82.80  
  Purchase #5 $127.79   12.526%   $143.797   $143.80   $143.80   $143.80  
  Purchase #6 $59.75   11.544%   $66.6475   $66.65   $66.65   $66.60  
  Purchase #7 $77.29   7.786%   $83.3078   $83.31   $83.30   $83.30  
  7 purchase totals = $487.92       $536.6798   $536.69   $536.70   $536.60