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 $123.91   12.668%   $139.6069   $139.61   $139.60   $139.60  
  Purchase #2 $58.82   13.216%   $66.5937   $66.59   $66.60   $66.60  
  Purchase #3 $249.55   4.471%   $260.7074   $260.71   $260.70   $260.70  
  Purchase #4 $82.94   13.169%   $93.8624   $93.86   $93.85   $93.90  
  Purchase #5 $101.53   11.383%   $113.0872   $113.09   $113.10   $113.10  
  Purchase #6 $219.81   11.643%   $245.4025   $245.40   $245.40   $245.40  
  Purchase #7 $121.37   12.693%   $136.7755   $136.78   $136.80   $136.80  
  7 purchase totals = $957.93       $1,056.0356   $1,056.04   $1,056.05   $1,056.10