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 $248.87   4.859%   $260.9626   $260.96   $260.95   $261.00  
  Purchase #2 $232.86   4.494%   $243.3247   $243.32   $243.30   $243.30  
  Purchase #3 $202.90   11.366%   $225.9616   $225.96   $225.95   $226.00  
  Purchase #4 $222.32   3.484%   $230.0656   $230.07   $230.05   $230.10  
  Purchase #5 $190.74   9.308%   $208.4941   $208.49   $208.50   $208.50  
  Purchase #6 $223.73   12.131%   $250.8707   $250.87   $250.85   $250.90  
  Purchase #7 $226.76   10.704%   $251.0324   $251.03   $251.05   $251.00  
  7 purchase totals = $1,548.18       $1,670.7117   $1,670.70   $1,670.65   $1,670.80