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 $17.87   9.112%   $19.4983   $19.50   $19.50   $19.50  
  Purchase #2 $18.59   6.511%   $19.8004   $19.80   $19.80   $19.80  
  Purchase #3 $112.98   5.367%   $119.0436   $119.04   $119.05   $119.00  
  Purchase #4 $139.30   5.266%   $146.6355   $146.64   $146.65   $146.60  
  Purchase #5 $151.89   4.833%   $159.2308   $159.23   $159.25   $159.20  
  Purchase #6 $51.70   11.825%   $57.8135   $57.81   $57.80   $57.80  
  Purchase #7 $163.56   12.444%   $183.9134   $183.91   $183.90   $183.90  
  7 purchase totals = $655.89       $705.9355   $705.93   $705.95   $705.80