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 $81.49   10.150%   $89.7612   $89.76   $89.75   $89.80  
  Purchase #2 $15.27   10.476%   $16.8697   $16.87   $16.85   $16.90  
  Purchase #3 $161.64   12.471%   $181.7981   $181.80   $181.80   $181.80  
  Purchase #4 $173.33   3.110%   $178.7206   $178.72   $178.70   $178.70  
  Purchase #5 $209.62   10.509%   $231.649   $231.65   $231.65   $231.60  
  Purchase #6 $17.89   6.496%   $19.0521   $19.05   $19.05   $19.10  
  Purchase #7 $237.85   9.760%   $261.0642   $261.06   $261.05   $261.10  
  7 purchase totals = $897.09       $978.9149   $978.91   $978.85   $979.00