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 $18.44   8.430%   $19.9945   $19.99   $20.00   $20.00  
  Purchase #2 $70.81   13.522%   $80.3849   $80.38   $80.40   $80.40  
  Purchase #3 $60.63   10.475%   $66.981   $66.98   $67.00   $67.00  
  Purchase #4 $86.71   3.573%   $89.8081   $89.81   $89.80   $89.80  
  Purchase #5 $84.99   13.231%   $96.235   $96.24   $96.25   $96.20  
  Purchase #6 $176.51   6.126%   $187.323   $187.32   $187.30   $187.30  
  Purchase #7 $148.25   4.696%   $155.2118   $155.21   $155.20   $155.20  
  7 purchase totals = $646.34       $695.9383   $695.93   $695.95   $695.90