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 $19.25   9.773%   $21.1313   $21.13   $21.15   $21.10  
  Purchase #2 $17.61   4.247%   $18.3579   $18.36   $18.35   $18.40  
  Purchase #3 $5.88   13.258%   $6.6596   $6.66   $6.65   $6.70  
  Purchase #4 $107.99   12.315%   $121.289   $121.29   $121.30   $121.30  
  Purchase #5 $189.92   13.614%   $215.7757   $215.78   $215.80   $215.80  
  Purchase #6 $21.70   7.846%   $23.4026   $23.40   $23.40   $23.40  
  Purchase #7 $245.79   13.604%   $279.2273   $279.23   $279.25   $279.20  
  7 purchase totals = $608.14       $685.8434   $685.85   $685.90   $685.90