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 $240.99   8.492%   $261.4549   $261.45   $261.45   $261.50  
  Purchase #2 $95.39   12.521%   $107.3338   $107.33   $107.35   $107.30  
  Purchase #3 $171.34   5.819%   $181.3103   $181.31   $181.30   $181.30  
  Purchase #4 $92.76   8.525%   $100.6678   $100.67   $100.65   $100.70  
  Purchase #5 $48.78   8.488%   $52.9204   $52.92   $52.90   $52.90  
  Purchase #6 $220.76   5.418%   $232.7208   $232.72   $232.70   $232.70  
  Purchase #7 $81.61   3.380%   $84.3684   $84.37   $84.35   $84.40  
  7 purchase totals = $951.63       $1,020.7764   $1,020.77   $1,020.70   $1,020.80