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 $39.86   4.120%   $41.5022   $41.50   $41.50   $41.50  
  Purchase #2 $141.30   11.419%   $157.435   $157.44   $157.45   $157.40  
  Purchase #3 $84.71   8.672%   $92.0561   $92.06   $92.05   $92.10  
  Purchase #4 $234.75   10.590%   $259.61   $259.61   $259.60   $259.60  
  Purchase #5 $52.80   11.572%   $58.91   $58.91   $58.90   $58.90  
  Purchase #6 $160.27   10.143%   $176.5262   $176.53   $176.55   $176.50  
  Purchase #7 $244.44   3.474%   $252.9318   $252.93   $252.95   $252.90  
  7 purchase totals = $958.13       $1,038.9713   $1,038.98   $1,039.00   $1,038.90