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 $188.94   9.546%   $206.9762   $206.98   $207.00   $207.00  
  Purchase #2 $66.85   13.147%   $75.6388   $75.64   $75.65   $75.60  
  Purchase #3 $159.30   3.190%   $164.3817   $164.38   $164.40   $164.40  
  Purchase #4 $101.72   6.412%   $108.2423   $108.24   $108.25   $108.20  
  Purchase #5 $116.67   4.524%   $121.9482   $121.95   $121.95   $121.90  
  Purchase #6 $207.83   13.781%   $236.4711   $236.47   $236.45   $236.50  
  Purchase #7 $72.96   4.772%   $76.4417   $76.44   $76.45   $76.40  
  7 purchase totals = $914.27       $990.1000   $990.10   $990.15   $990.00