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 $247.27   13.458%   $280.5476   $280.55   $280.55   $280.50  
  Purchase #2 $34.83   3.292%   $35.9766   $35.98   $36.00   $36.00  
  Purchase #3 $86.76   12.544%   $97.6432   $97.64   $97.65   $97.60  
  Purchase #4 $26.39   5.316%   $27.7929   $27.79   $27.80   $27.80  
  Purchase #5 $97.97   11.814%   $109.5442   $109.54   $109.55   $109.50  
  Purchase #6 $51.38   5.617%   $54.266   $54.27   $54.25   $54.30  
  Purchase #7 $242.93   6.116%   $257.7876   $257.79   $257.80   $257.80  
  7 purchase totals = $787.53       $863.5581   $863.56   $863.60   $863.50