Yes and no. First of all, a stock ECU on a car with NO modifications would still benefit from an ECU tune.
If you do have any drivetrain modifications, the tune is really the only way to take full advantage. You will get partial improvement if you do nothing but bolt-on mods with no tune.
Resetting the ECU does not take the place of a tune. All it does is zero out the ECU learning cells, forcing the computer to learn all over again. After any breathing mod (intake, MAF, Throttle Body, Plenum, Exhaust) resetting the ECU will benefit performance, but only after the computer has had a chance to "learn" the new data, typically about a tank of gas. So yes, you should absolutely reset your ECU after a CAI or plenum spacer. It can never harm your FX to reset the ECU at any time.
On top of that, you should also get a tune like Uprev or Technosquare, but I would recommend a dyno tune and not the "canned" ECU tune. For each of my mods I reset the ECU and noticed improvements in MPG and feel after a week or so of driving. It was only after the Uprev tune that my FX really showed new life.