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 $66.29   11.329%   $73.8   $73.80   $73.80   $73.80  
  Purchase #2 $31.98   13.339%   $36.2458   $36.25   $36.25   $36.20  
  Purchase #3 $157.33   7.712%   $169.4633   $169.46   $169.45   $169.50  
  Purchase #4 $26.37   7.693%   $28.3986   $28.40   $28.40   $28.40  
  Purchase #5 $227.54   4.533%   $237.8544   $237.85   $237.85   $237.90  
  Purchase #6 $26.69   3.322%   $27.5766   $27.58   $27.60   $27.60  
  Purchase #7 $250.88   3.863%   $260.5715   $260.57   $260.55   $260.60  
  7 purchase totals = $787.08       $833.9102   $833.91   $833.90   $834.00