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 $226.71   11.805%   $253.4731   $253.47   $253.45   $253.50  
  Purchase #2 $21.26   12.461%   $23.9092   $23.91   $23.90   $23.90  
  Purchase #3 $132.86   6.209%   $141.1093   $141.11   $141.10   $141.10  
  Purchase #4 $49.52   7.348%   $53.1587   $53.16   $53.15   $53.20  
  Purchase #5 $205.70   4.491%   $214.938   $214.94   $214.95   $214.90  
  Purchase #6 $103.41   5.165%   $108.7511   $108.75   $108.75   $108.80  
  Purchase #7 $103.49   7.257%   $111.0003   $111.00   $111.00   $111.00  
  7 purchase totals = $842.95       $906.3397   $906.34   $906.30   $906.40