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 $160.98   13.806%   $183.2049   $183.20   $183.20   $183.20  
  Purchase #2 $115.75   13.630%   $131.5267   $131.53   $131.55   $131.50  
  Purchase #3 $27.69   5.408%   $29.1875   $29.19   $29.20   $29.20  
  Purchase #4 $214.92   11.854%   $240.3966   $240.40   $240.40   $240.40  
  Purchase #5 $91.52   13.817%   $104.1653   $104.17   $104.15   $104.20  
  Purchase #6 $186.86   3.770%   $193.9046   $193.90   $193.90   $193.90  
  Purchase #7 $34.65   3.582%   $35.8912   $35.89   $35.90   $35.90  
  7 purchase totals = $832.37       $918.2768   $918.28   $918.30   $918.30