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 $10.79   6.259%   $11.4653   $11.47   $11.45   $11.50  
  Purchase #2 $51.36   13.561%   $58.3249   $58.32   $58.30   $58.30  
  Purchase #3 $121.98   3.683%   $126.4725   $126.47   $126.45   $126.50  
  Purchase #4 $11.66   11.867%   $13.0437   $13.04   $13.05   $13.00  
  Purchase #5 $134.82   3.664%   $139.7598   $139.76   $139.75   $139.80  
  Purchase #6 $136.67   6.610%   $145.7039   $145.70   $145.70   $145.70  
  Purchase #7 $31.51   5.564%   $33.2632   $33.26   $33.25   $33.30  
  7 purchase totals = $498.79       $528.0333   $528.02   $527.95   $528.10