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 $30.49   12.520%   $34.3073   $34.31   $34.30   $34.30  
  Purchase #2 $213.85   12.860%   $241.3511   $241.35   $241.35   $241.40  
  Purchase #3 $239.86   9.102%   $261.6921   $261.69   $261.70   $261.70  
  Purchase #4 $159.38   12.167%   $178.7718   $178.77   $178.75   $178.80  
  Purchase #5 $90.94   6.382%   $96.7438   $96.74   $96.75   $96.70  
  Purchase #6 $195.25   5.838%   $206.6487   $206.65   $206.65   $206.60  
  Purchase #7 $75.27   7.265%   $80.7384   $80.74   $80.75   $80.70  
  7 purchase totals = $1,005.04       $1,100.2532   $1,100.25   $1,100.25   $1,100.20