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 $58.82   13.503%   $66.7625   $66.76   $66.75   $66.80  
  Purchase #2 $52.37   12.521%   $58.9272   $58.93   $58.95   $58.90  
  Purchase #3 $128.81   3.387%   $133.1728   $133.17   $133.15   $133.20  
  Purchase #4 $217.87   12.846%   $245.8576   $245.86   $245.85   $245.90  
  Purchase #5 $7.58   3.693%   $7.8599   $7.86   $7.85   $7.90  
  Purchase #6 $48.79   6.347%   $51.8867   $51.89   $51.90   $51.90  
  Purchase #7 $250.63   3.751%   $260.0311   $260.03   $260.05   $260.00  
  7 purchase totals = $764.87       $824.4978   $824.50   $824.50   $824.60