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 $6.90   12.767%   $7.7809   $7.78   $7.80   $7.80  
  Purchase #2 $37.88   8.491%   $41.0964   $41.10   $41.10   $41.10  
  Purchase #3 $88.40   4.329%   $92.2268   $92.23   $92.25   $92.20  
  Purchase #4 $245.65   9.374%   $268.6772   $268.68   $268.70   $268.70  
  Purchase #5 $30.60   12.635%   $34.4663   $34.47   $34.45   $34.50  
  Purchase #6 $209.41   13.874%   $238.4635   $238.46   $238.45   $238.50  
  Purchase #7 $221.71   11.172%   $246.4794   $246.48   $246.50   $246.50  
  7 purchase totals = $840.55       $929.1905   $929.20   $929.25   $929.30