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 $122.86   4.426%   $128.2978   $128.30   $128.30   $128.30  
  Purchase #2 $49.77   13.515%   $56.4964   $56.50   $56.50   $56.50  
  Purchase #3 $181.25   5.477%   $191.1771   $191.18   $191.20   $191.20  
  Purchase #4 $23.46   9.243%   $25.6284   $25.63   $25.65   $25.60  
  Purchase #5 $82.44   9.795%   $90.515   $90.52   $90.50   $90.50  
  Purchase #6 $56.44   7.604%   $60.7317   $60.73   $60.75   $60.70  
  Purchase #7 $84.58   3.504%   $87.5437   $87.54   $87.55   $87.50  
  7 purchase totals = $600.80       $640.3901   $640.40   $640.45   $640.30