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 $183.32   3.307%   $189.3824   $189.38   $189.40   $189.40  
  Purchase #2 $161.97   5.215%   $170.4167   $170.42   $170.40   $170.40  
  Purchase #3 $186.57   10.847%   $206.8072   $206.81   $206.80   $206.80  
  Purchase #4 $232.50   10.558%   $257.0474   $257.05   $257.05   $257.00  
  Purchase #5 $156.51   10.463%   $172.8856   $172.89   $172.90   $172.90  
  Purchase #6 $97.48   6.345%   $103.6651   $103.67   $103.65   $103.70  
  Purchase #7 $240.32   13.762%   $273.3928   $273.39   $273.40   $273.40  
  7 purchase totals = $1,258.67       $1,373.5972   $1,373.61   $1,373.60   $1,373.60