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 $28.34   3.824%   $29.4237   $29.42   $29.40   $29.40  
  Purchase #2 $96.80   11.566%   $107.9959   $108.00   $108.00   $108.00  
  Purchase #3 $49.77   4.144%   $51.8325   $51.83   $51.85   $51.80  
  Purchase #4 $7.75   7.597%   $8.3388   $8.34   $8.35   $8.30  
  Purchase #5 $37.93   5.560%   $40.0389   $40.04   $40.05   $40.00  
  Purchase #6 $27.97   9.104%   $30.5164   $30.52   $30.50   $30.50  
  Purchase #7 $236.47   8.224%   $255.9173   $255.92   $255.90   $255.90  
  7 purchase totals = $485.03       $524.0635   $524.07   $524.05   $523.90