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 $246.83   10.464%   $272.6583   $272.66   $272.65   $272.70  
  Purchase #2 $227.36   12.618%   $256.0483   $256.05   $256.05   $256.00  
  Purchase #3 $151.46   11.181%   $168.3947   $168.39   $168.40   $168.40  
  Purchase #4 $49.56   8.426%   $53.7359   $53.74   $53.75   $53.70  
  Purchase #5 $110.85   5.390%   $116.8248   $116.82   $116.80   $116.80  
  Purchase #6 $19.70   9.182%   $21.5089   $21.51   $21.50   $21.50  
  Purchase #7 $46.95   6.354%   $49.9332   $49.93   $49.95   $49.90  
  7 purchase totals = $852.71       $939.1041   $939.10   $939.10   $939.00