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 $57.91   12.278%   $65.0202   $65.02   $65.00   $65.00  
  Purchase #2 $128.68   10.250%   $141.8697   $141.87   $141.85   $141.90  
  Purchase #3 $16.76   6.385%   $17.8301   $17.83   $17.85   $17.80  
  Purchase #4 $200.39   12.233%   $224.9037   $224.90   $224.90   $224.90  
  Purchase #5 $221.71   9.548%   $242.8789   $242.88   $242.90   $242.90  
  Purchase #6 $92.98   7.313%   $99.7796   $99.78   $99.80   $99.80  
  Purchase #7 $158.28   10.196%   $174.4182   $174.42   $174.40   $174.40  
  7 purchase totals = $876.71       $966.7004   $966.70   $966.70   $966.70