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 $19.42   8.809%   $21.1307   $21.13   $21.15   $21.10  
  Purchase #2 $121.60   12.768%   $137.1259   $137.13   $137.15   $137.10  
  Purchase #3 $15.83   12.762%   $17.8502   $17.85   $17.85   $17.90  
  Purchase #4 $142.78   4.843%   $149.6948   $149.69   $149.70   $149.70  
  Purchase #5 $85.61   3.369%   $88.4942   $88.49   $88.50   $88.50  
  Purchase #6 $35.77   7.178%   $38.3376   $38.34   $38.35   $38.30  
  Purchase #7 $197.30   11.156%   $219.3108   $219.31   $219.30   $219.30  
  7 purchase totals = $618.31       $671.9442   $671.94   $672.00   $671.90