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 $250.61   13.112%   $283.47   $283.47   $283.45   $283.50  
  Purchase #2 $67.98   8.537%   $73.7835   $73.78   $73.80   $73.80  
  Purchase #3 $194.85   13.848%   $221.8328   $221.83   $221.85   $221.80  
  Purchase #4 $150.63   9.602%   $165.0935   $165.09   $165.10   $165.10  
  Purchase #5 $177.57   6.103%   $188.4071   $188.41   $188.40   $188.40  
  Purchase #6 $164.60   5.553%   $173.7402   $173.74   $173.75   $173.70  
  Purchase #7 $199.74   6.837%   $213.3962   $213.40   $213.40   $213.40  
  7 purchase totals = $1,205.98       $1,319.7233   $1,319.72   $1,319.75   $1,319.70