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 $96.63   4.771%   $101.2402   $101.24   $101.25   $101.20  
  Purchase #2 $19.46   13.755%   $22.1367   $22.14   $22.15   $22.10  
  Purchase #3 $75.67   6.131%   $80.3093   $80.31   $80.30   $80.30  
  Purchase #4 $169.49   5.604%   $178.9882   $178.99   $179.00   $179.00  
  Purchase #5 $234.91   13.675%   $267.0339   $267.03   $267.05   $267.00  
  Purchase #6 $59.61   9.462%   $65.2503   $65.25   $65.25   $65.30  
  Purchase #7 $52.80   12.405%   $59.3498   $59.35   $59.35   $59.30  
  7 purchase totals = $708.57       $774.3084   $774.31   $774.35   $774.20