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 $5.60   12.743%   $6.3136   $6.31   $6.30   $6.30  
  Purchase #2 $45.61   5.239%   $47.9995   $48.00   $48.00   $48.00  
  Purchase #3 $6.73   9.775%   $7.3879   $7.39   $7.40   $7.40  
  Purchase #4 $230.91   10.265%   $254.6129   $254.61   $254.60   $254.60  
  Purchase #5 $124.73   13.705%   $141.8242   $141.82   $141.80   $141.80  
  Purchase #6 $16.71   9.415%   $18.2832   $18.28   $18.30   $18.30  
  Purchase #7 $14.41   13.208%   $16.3133   $16.31   $16.30   $16.30  
  7 purchase totals = $444.70       $492.7346   $492.72   $492.70   $492.70