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 $120.57   3.855%   $125.218   $125.22   $125.20   $125.20  
  Purchase #2 $29.52   4.700%   $30.9074   $30.91   $30.90   $30.90  
  Purchase #3 $14.92   3.795%   $15.4862   $15.49   $15.50   $15.50  
  Purchase #4 $36.97   9.397%   $40.4441   $40.44   $40.45   $40.40  
  Purchase #5 $227.74   7.769%   $245.4331   $245.43   $245.45   $245.40  
  Purchase #6 $141.29   10.428%   $156.0237   $156.02   $156.00   $156.00  
  Purchase #7 $25.77   4.748%   $26.9936   $26.99   $27.00   $27.00  
  7 purchase totals = $596.78       $640.5061   $640.50   $640.50   $640.40