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 $116.77   10.548%   $129.0869   $129.09   $129.10   $129.10  
  Purchase #2 $153.48   3.103%   $158.2425   $158.24   $158.25   $158.20  
  Purchase #3 $239.84   4.632%   $250.9494   $250.95   $250.95   $250.90  
  Purchase #4 $19.51   7.595%   $20.9918   $20.99   $21.00   $21.00  
  Purchase #5 $77.89   7.531%   $83.7559   $83.76   $83.75   $83.80  
  Purchase #6 $42.28   8.690%   $45.9541   $45.95   $45.95   $46.00  
  Purchase #7 $220.79   4.802%   $231.3923   $231.39   $231.40   $231.40  
  7 purchase totals = $870.56       $920.3729   $920.37   $920.40   $920.40