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 $154.29   8.165%   $166.8878   $166.89   $166.90   $166.90  
  Purchase #2 $79.40   12.307%   $89.1718   $89.17   $89.15   $89.20  
  Purchase #3 $241.44   10.700%   $267.2741   $267.27   $267.25   $267.30  
  Purchase #4 $182.32   3.863%   $189.363   $189.36   $189.35   $189.40  
  Purchase #5 $199.51   8.625%   $216.7177   $216.72   $216.70   $216.70  
  Purchase #6 $9.29   13.654%   $10.5585   $10.56   $10.55   $10.60  
  Purchase #7 $220.88   12.753%   $249.0488   $249.05   $249.05   $249.00  
  7 purchase totals = $1,087.13       $1,189.0217   $1,189.02   $1,188.95   $1,189.10