A re-used screw top does not provide a completely airtight seal, thats not a major problem for short term storage but anything kept for a long time will suffer with oxygenation which leads to loss of quality.
One potential issue I should mention is that the necks of screw top bottles are weaker than bottles intended to take corks, depending on the type of corker you use you can break the bottles while inserting a cork.
Some corkers grip the neck of the bottle to supply the upward force while pushing down on the cork with a plunger, these should be avoided with screw top bottles. I use a floor corker which eliminates this issue.
