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 $86.74   7.210%   $92.994   $92.99   $93.00   $93.00  
  Purchase #2 $22.80   4.856%   $23.9072   $23.91   $23.90   $23.90  
  Purchase #3 $120.73   13.246%   $136.7219   $136.72   $136.70   $136.70  
  Purchase #4 $71.74   8.775%   $78.0352   $78.04   $78.05   $78.00  
  Purchase #5 $109.73   5.653%   $115.933   $115.93   $115.95   $115.90  
  Purchase #6 $59.57   6.618%   $63.5123   $63.51   $63.50   $63.50  
  Purchase #7 $73.30   10.761%   $81.1878   $81.19   $81.20   $81.20  
  7 purchase totals = $544.61       $592.2914   $592.29   $592.30   $592.20