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 $38.67   10.316%   $42.6592   $42.66   $42.65   $42.70  
  Purchase #2 $57.91   3.264%   $59.8002   $59.80   $59.80   $59.80  
  Purchase #3 $240.90   9.529%   $263.8554   $263.86   $263.85   $263.90  
  Purchase #4 $66.94   4.124%   $69.7006   $69.70   $69.70   $69.70  
  Purchase #5 $214.97   13.221%   $243.3912   $243.39   $243.40   $243.40  
  Purchase #6 $190.25   5.115%   $199.9813   $199.98   $200.00   $200.00  
  Purchase #7 $97.75   9.268%   $106.8095   $106.81   $106.80   $106.80  
  7 purchase totals = $907.39       $986.1974   $986.20   $986.20   $986.30