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 $221.45   12.444%   $249.0072   $249.01   $249.00   $249.00  
  Purchase #2 $106.25   5.377%   $111.9631   $111.96   $111.95   $112.00  
  Purchase #3 $235.70   11.246%   $262.2068   $262.21   $262.20   $262.20  
  Purchase #4 $234.57   8.238%   $253.8939   $253.89   $253.90   $253.90  
  Purchase #5 $196.76   11.381%   $219.1533   $219.15   $219.15   $219.20  
  Purchase #6 $145.63   9.832%   $159.9483   $159.95   $159.95   $159.90  
  Purchase #7 $58.30   8.439%   $63.2199   $63.22   $63.20   $63.20  
  7 purchase totals = $1,198.66       $1,319.3925   $1,319.39   $1,319.35   $1,319.40