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 $37.91   6.306%   $40.3006   $40.30   $40.30   $40.30  
  Purchase #2 $185.94   6.342%   $197.7323   $197.73   $197.75   $197.70  
  Purchase #3 $43.50   7.358%   $46.7007   $46.70   $46.70   $46.70  
  Purchase #4 $91.68   5.793%   $96.991   $96.99   $97.00   $97.00  
  Purchase #5 $67.68   8.231%   $73.2507   $73.25   $73.25   $73.30  
  Purchase #6 $132.47   5.299%   $139.4896   $139.49   $139.50   $139.50  
  Purchase #7 $57.58   12.830%   $64.9675   $64.97   $64.95   $65.00  
  7 purchase totals = $616.76       $659.4324   $659.43   $659.45   $659.50