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 $16.90   6.487%   $17.9963   $18.00   $18.00   $18.00  
  Purchase #2 $140.70   7.381%   $151.0851   $151.09   $151.10   $151.10  
  Purchase #3 $77.38   5.249%   $81.4417   $81.44   $81.45   $81.40  
  Purchase #4 $17.66   13.523%   $20.0482   $20.05   $20.05   $20.00  
  Purchase #5 $159.28   11.832%   $178.126   $178.13   $178.15   $178.10  
  Purchase #6 $92.88   7.834%   $100.1562   $100.16   $100.15   $100.20  
  Purchase #7 $125.81   12.598%   $141.6595   $141.66   $141.65   $141.70  
  7 purchase totals = $630.61       $690.5130   $690.53   $690.55   $690.50