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 $15.75   9.539%   $17.2524   $17.25   $17.25   $17.30  
  Purchase #2 $202.87   10.279%   $223.723   $223.72   $223.70   $223.70  
  Purchase #3 $167.40   10.142%   $184.3777   $184.38   $184.40   $184.40  
  Purchase #4 $6.87   7.489%   $7.3845   $7.38   $7.40   $7.40  
  Purchase #5 $242.35   9.844%   $266.2069   $266.21   $266.20   $266.20  
  Purchase #6 $110.78   9.389%   $121.1811   $121.18   $121.20   $121.20  
  Purchase #7 $144.89   9.800%   $159.0892   $159.09   $159.10   $159.10  
  7 purchase totals = $890.91       $979.2148   $979.21   $979.25   $979.30