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 $139.70   7.770%   $150.5547   $150.55   $150.55   $150.60  
  Purchase #2 $78.59   7.232%   $84.2736   $84.27   $84.25   $84.30  
  Purchase #3 $192.63   13.573%   $218.7757   $218.78   $218.80   $218.80  
  Purchase #4 $224.63   6.662%   $239.5949   $239.59   $239.60   $239.60  
  Purchase #5 $200.45   11.104%   $222.708   $222.71   $222.70   $222.70  
  Purchase #6 $18.26   10.441%   $20.1665   $20.17   $20.15   $20.20  
  Purchase #7 $30.89   3.505%   $31.9727   $31.97   $31.95   $32.00  
  7 purchase totals = $885.15       $968.0461   $968.04   $968.00   $968.20