1996 Sebring Converable, 2.5l, 98K miles. Having a hard start (cold engine), about 2-5 sec cranking (used to start within 1-2 sec of cranking), once started a rough idle. The idle gets better after the engine warms up. No ignition miss, or fault codes. Plugs and wires, cap were replaced in June'07 approx 93K miles.

Sounds like your IAC valve may need cleaning.

I did have the intake plenum off to replace the wires and plugs. This symptom (hard start, poor idle) started about 1 week afterwards. As of today, it started within 2-3 sec, idle is acceptable, considering 98K miles. Recently added about 10 gal of fuel.

I re-checked that all the vacuum and electrical connections were on and tight.

Would putting a bottle of fuel injector cleaner cause a hard start condidtion (prior to putting in fresh fuel)?

Fuel injector cleaner shouldn't cause an issue. You can also spray some throttle body cleaner down it's throat.