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 $121.92   6.619%   $129.9899   $129.99   $130.00   $130.00  
  Purchase #2 $75.60   3.331%   $78.1182   $78.12   $78.10   $78.10  
  Purchase #3 $90.81   8.192%   $98.2492   $98.25   $98.25   $98.20  
  Purchase #4 $112.52   12.103%   $126.1383   $126.14   $126.15   $126.10  
  Purchase #5 $235.54   12.379%   $264.6975   $264.70   $264.70   $264.70  
  Purchase #6 $146.49   7.310%   $157.1984   $157.20   $157.20   $157.20  
  Purchase #7 $62.53   11.803%   $69.9104   $69.91   $69.90   $69.90  
  7 purchase totals = $845.41       $924.3019   $924.31   $924.30   $924.20