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 $139.61   5.545%   $147.3514   $147.35   $147.35   $147.40  
  Purchase #2 $143.59   13.533%   $163.022   $163.02   $163.00   $163.00  
  Purchase #3 $235.99   13.716%   $268.3584   $268.36   $268.35   $268.40  
  Purchase #4 $38.70   3.517%   $40.0611   $40.06   $40.05   $40.10  
  Purchase #5 $21.67   4.374%   $22.6178   $22.62   $22.60   $22.60  
  Purchase #6 $228.94   8.709%   $248.8784   $248.88   $248.90   $248.90  
  Purchase #7 $93.93   5.317%   $98.9243   $98.92   $98.90   $98.90  
  7 purchase totals = $902.43       $989.2134   $989.21   $989.15   $989.30