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 $56.42   13.634%   $64.1123   $64.11   $64.10   $64.10  
  Purchase #2 $133.88   10.302%   $147.6723   $147.67   $147.65   $147.70  
  Purchase #3 $18.53   10.340%   $20.446   $20.45   $20.45   $20.40  
  Purchase #4 $16.34   7.587%   $17.5797   $17.58   $17.60   $17.60  
  Purchase #5 $96.80   5.591%   $102.2121   $102.21   $102.20   $102.20  
  Purchase #6 $112.29   7.747%   $120.9891   $120.99   $121.00   $121.00  
  Purchase #7 $146.60   10.369%   $161.801   $161.80   $161.80   $161.80  
  7 purchase totals = $580.86       $634.8125   $634.81   $634.80   $634.80