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 $18.70   8.602%   $20.3086   $20.31   $20.30   $20.30  
  Purchase #2 $31.80   13.535%   $36.1041   $36.10   $36.10   $36.10  
  Purchase #3 $49.48   13.739%   $56.2781   $56.28   $56.30   $56.30  
  Purchase #4 $105.87   7.413%   $113.7181   $113.72   $113.70   $113.70  
  Purchase #5 $250.57   4.626%   $262.1614   $262.16   $262.15   $262.20  
  Purchase #6 $196.61   7.265%   $210.8937   $210.89   $210.90   $210.90  
  Purchase #7 $180.84   6.301%   $192.2347   $192.23   $192.25   $192.20  
  7 purchase totals = $833.87       $891.6987   $891.69   $891.70   $891.70