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 $8.61   4.244%   $8.9754   $8.98   $9.00   $9.00  
  Purchase #2 $42.30   5.502%   $44.6273   $44.63   $44.65   $44.60  
  Purchase #3 $179.41   13.273%   $203.2231   $203.22   $203.20   $203.20  
  Purchase #4 $132.41   9.496%   $144.9837   $144.98   $145.00   $145.00  
  Purchase #5 $122.30   3.804%   $126.9523   $126.95   $126.95   $127.00  
  Purchase #6 $199.60   10.180%   $219.9193   $219.92   $219.90   $219.90  
  Purchase #7 $87.75   9.553%   $96.1328   $96.13   $96.15   $96.10  
  7 purchase totals = $772.38       $844.8139   $844.81   $844.85   $844.80