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 $72.78   11.286%   $80.994   $80.99   $81.00   $81.00  
  Purchase #2 $17.52   13.549%   $19.8938   $19.89   $19.90   $19.90  
  Purchase #3 $240.41   3.169%   $248.0286   $248.03   $248.05   $248.00  
  Purchase #4 $199.40   10.278%   $219.8943   $219.89   $219.90   $219.90  
  Purchase #5 $52.50   8.769%   $57.1037   $57.10   $57.10   $57.10  
  Purchase #6 $112.91   7.615%   $121.5081   $121.51   $121.50   $121.50  
  Purchase #7 $164.73   3.209%   $170.0162   $170.02   $170.00   $170.00  
  7 purchase totals = $860.25       $917.4387   $917.43   $917.45   $917.40