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 $77.99   7.334%   $83.7098   $83.71   $83.70   $83.70  
  Purchase #2 $38.64   3.667%   $40.0569   $40.06   $40.05   $40.10  
  Purchase #3 $13.39   4.573%   $14.0023   $14.00   $14.00   $14.00  
  Purchase #4 $190.38   10.729%   $210.8059   $210.81   $210.80   $210.80  
  Purchase #5 $25.26   10.657%   $27.952   $27.95   $27.95   $28.00  
  Purchase #6 $226.75   9.440%   $248.1552   $248.16   $248.15   $248.20  
  Purchase #7 $194.91   11.491%   $217.3071   $217.31   $217.30   $217.30  
  7 purchase totals = $767.32       $841.9892   $842.00   $841.95   $842.10