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 $30.87   11.603%   $34.4518   $34.45   $34.45   $34.50  
  Purchase #2 $62.57   3.782%   $64.9364   $64.94   $64.95   $64.90  
  Purchase #3 $217.58   4.306%   $226.949   $226.95   $226.95   $226.90  
  Purchase #4 $29.47   4.213%   $30.7116   $30.71   $30.70   $30.70  
  Purchase #5 $196.73   12.782%   $221.876   $221.88   $221.90   $221.90  
  Purchase #6 $163.93   5.421%   $172.8166   $172.82   $172.80   $172.80  
  Purchase #7 $229.97   4.260%   $239.7667   $239.77   $239.75   $239.80  
  7 purchase totals = $931.12       $991.5081   $991.52   $991.50   $991.50