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 $121.77   6.540%   $129.7338   $129.73   $129.75   $129.70  
  Purchase #2 $174.82   13.310%   $198.0885   $198.09   $198.10   $198.10  
  Purchase #3 $90.46   6.243%   $96.1074   $96.11   $96.10   $96.10  
  Purchase #4 $69.95   11.513%   $78.0033   $78.00   $78.00   $78.00  
  Purchase #5 $55.95   5.463%   $59.0065   $59.01   $59.00   $59.00  
  Purchase #6 $16.25   8.460%   $17.6248   $17.62   $17.60   $17.60  
  Purchase #7 $52.37   11.430%   $58.3559   $58.36   $58.35   $58.40  
  7 purchase totals = $581.57       $636.9202   $636.92   $636.90   $636.90