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 $19.48   7.857%   $21.0105   $21.01   $21.00   $21.00  
  Purchase #2 $24.81   7.347%   $26.6328   $26.63   $26.65   $26.60  
  Purchase #3 $110.50   13.163%   $125.0451   $125.05   $125.05   $125.00  
  Purchase #4 $216.61   6.166%   $229.9662   $229.97   $229.95   $230.00  
  Purchase #5 $87.74   4.566%   $91.7462   $91.75   $91.75   $91.70  
  Purchase #6 $85.99   4.133%   $89.544   $89.54   $89.55   $89.50  
  Purchase #7 $52.95   6.284%   $56.2774   $56.28   $56.30   $56.30  
  7 purchase totals = $598.08       $640.2222   $640.23   $640.25   $640.10