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 $221.37   12.266%   $248.5232   $248.52   $248.50   $248.50  
  Purchase #2 $240.59   11.743%   $268.8425   $268.84   $268.85   $268.80  
  Purchase #3 $64.44   7.486%   $69.264   $69.26   $69.25   $69.30  
  Purchase #4 $110.30   8.718%   $119.916   $119.92   $119.90   $119.90  
  Purchase #5 $114.36   3.782%   $118.6851   $118.69   $118.70   $118.70  
  Purchase #6 $18.46   12.756%   $20.8148   $20.81   $20.80   $20.80  
  Purchase #7 $15.47   5.319%   $16.2928   $16.29   $16.30   $16.30  
  7 purchase totals = $784.99       $862.3384   $862.33   $862.30   $862.30