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 $5.86   13.814%   $6.6695   $6.67   $6.65   $6.70  
  Purchase #2 $58.58   6.874%   $62.6068   $62.61   $62.60   $62.60  
  Purchase #3 $87.56   11.567%   $97.6881   $97.69   $97.70   $97.70  
  Purchase #4 $222.91   13.407%   $252.7955   $252.80   $252.80   $252.80  
  Purchase #5 $23.75   6.704%   $25.3422   $25.34   $25.35   $25.30  
  Purchase #6 $247.98   5.759%   $262.2612   $262.26   $262.25   $262.30  
  Purchase #7 $223.89   11.495%   $249.6262   $249.63   $249.65   $249.60  
  7 purchase totals = $870.53       $956.9895   $957.00   $957.00   $957.00