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 $247.61   13.873%   $281.9609   $281.96   $281.95   $282.00  
  Purchase #2 $183.69   6.383%   $195.4149   $195.41   $195.40   $195.40  
  Purchase #3 $16.61   11.153%   $18.4625   $18.46   $18.45   $18.50  
  Purchase #4 $229.35   12.453%   $257.911   $257.91   $257.90   $257.90  
  Purchase #5 $32.36   8.780%   $35.2012   $35.20   $35.20   $35.20  
  Purchase #6 $90.66   12.832%   $102.2935   $102.29   $102.30   $102.30  
  Purchase #7 $50.30   6.267%   $53.4523   $53.45   $53.45   $53.50  
  7 purchase totals = $850.58       $944.6963   $944.68   $944.65   $944.80