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 $8.99   10.408%   $9.9257   $9.93   $9.95   $9.90  
  Purchase #2 $101.85   6.301%   $108.2676   $108.27   $108.25   $108.30  
  Purchase #3 $60.50   8.341%   $65.5463   $65.55   $65.55   $65.50  
  Purchase #4 $136.90   8.496%   $148.531   $148.53   $148.55   $148.50  
  Purchase #5 $111.77   7.252%   $119.8756   $119.88   $119.90   $119.90  
  Purchase #6 $29.88   5.250%   $31.4487   $31.45   $31.45   $31.40  
  Purchase #7 $50.67   13.329%   $57.4238   $57.42   $57.40   $57.40  
  7 purchase totals = $500.56       $541.0187   $541.03   $541.05   $540.90