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 $40.27   5.648%   $42.5444   $42.54   $42.55   $42.50  
  Purchase #2 $216.81   11.590%   $241.9383   $241.94   $241.95   $241.90  
  Purchase #3 $124.60   8.142%   $134.7449   $134.74   $134.75   $134.70  
  Purchase #4 $221.66   13.354%   $251.2605   $251.26   $251.25   $251.30  
  Purchase #5 $8.64   3.724%   $8.9618   $8.96   $8.95   $9.00  
  Purchase #6 $75.87   5.450%   $80.0049   $80.00   $80.00   $80.00  
  Purchase #7 $214.80   5.405%   $226.4099   $226.41   $226.40   $226.40  
  7 purchase totals = $902.65       $985.8647   $985.85   $985.85   $985.80