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 $28.38   9.183%   $30.9861   $30.99   $31.00   $31.00  
  Purchase #2 $15.78   12.817%   $17.8025   $17.80   $17.80   $17.80  
  Purchase #3 $81.77   3.308%   $84.475   $84.48   $84.50   $84.50  
  Purchase #4 $154.76   4.503%   $161.7288   $161.73   $161.75   $161.70  
  Purchase #5 $182.45   7.440%   $196.0243   $196.02   $196.00   $196.00  
  Purchase #6 $229.98   12.627%   $259.0196   $259.02   $259.00   $259.00  
  Purchase #7 $24.99   9.104%   $27.2651   $27.27   $27.25   $27.30  
  7 purchase totals = $718.11       $777.3014   $777.31   $777.30   $777.30