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 $26.58   13.541%   $30.1792   $30.18   $30.20   $30.20  
  Purchase #2 $232.99   11.357%   $259.4507   $259.45   $259.45   $259.50  
  Purchase #3 $125.49   11.524%   $139.9515   $139.95   $139.95   $140.00  
  Purchase #4 $107.53   12.305%   $120.7616   $120.76   $120.75   $120.80  
  Purchase #5 $64.61   13.219%   $73.1508   $73.15   $73.15   $73.20  
  Purchase #6 $172.40   3.233%   $177.9737   $177.97   $177.95   $178.00  
  Purchase #7 $238.30   6.766%   $254.4234   $254.42   $254.40   $254.40  
  7 purchase totals = $967.90       $1,055.8909   $1,055.88   $1,055.85   $1,056.10