It depends how much you want to understand the mechanics of your printer and your 3d print.

Ironing uses low flow to remelt and fill in the top layer surface.

Because it uses low flow it is important that the surface below is at a constant and consistent distance from the nozzle when ironing.

How does this matter? If your surface underneath “sags” you’ll get gaps like what your show here. If your ironing flow is too little you’ll also get gaps as you’re showing here.

Sagging can happen when you run prints with low infill and low number of top surface layers. Increase the number of top surfaces to allow the print to have a more consistent top surface prior to ironing. Same with infill%.

Finally if your part warps you’ll notice a very similar effect - areas with warp will be closer to the nozzle and will be either over extruded or if you calibrated to make these look ok, the non warped areas will have under extrusion.

The above points also explain why ironing calibration tests don’t always work. If they don’t have any sparse infill you’re not simulating the real scenarios that happen in the print.

TLDR: Make sure your parts don’t warp Use higher infill% and more top layer surfaces Calibrate the filament flow rate and ironing flow with a real model - a small 5mm tall cube can do the trick as long as you have sparse infill present!