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 $221.99   12.186%   $249.0417   $249.04   $249.05   $249.00  
  Purchase #2 $162.68   13.506%   $184.6516   $184.65   $184.65   $184.70  
  Purchase #3 $212.93   3.448%   $220.2718   $220.27   $220.25   $220.30  
  Purchase #4 $139.69   13.737%   $158.8792   $158.88   $158.90   $158.90  
  Purchase #5 $103.84   11.688%   $115.9768   $115.98   $116.00   $116.00  
  Purchase #6 $233.29   8.841%   $253.9152   $253.92   $253.90   $253.90  
  Purchase #7 $187.67   6.309%   $199.5101   $199.51   $199.50   $199.50  
  7 purchase totals = $1,262.09       $1,382.2464   $1,382.25   $1,382.25   $1,382.30