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 $121.46   5.116%   $127.6739   $127.67   $127.65   $127.70  
  Purchase #2 $56.94   3.192%   $58.7575   $58.76   $58.75   $58.80  
  Purchase #3 $68.32   7.254%   $73.2759   $73.28   $73.30   $73.30  
  Purchase #4 $30.50   9.554%   $33.414   $33.41   $33.40   $33.40  
  Purchase #5 $129.66   6.659%   $138.2941   $138.29   $138.30   $138.30  
  Purchase #6 $246.25   11.676%   $275.0022   $275.00   $275.00   $275.00  
  Purchase #7 $104.63   12.305%   $117.5047   $117.50   $117.50   $117.50  
  7 purchase totals = $757.76       $823.9223   $823.91   $823.90   $824.00