For an ordinary road car, maintaining the correct tyre pressure is more important than paying for nitrogen rather than ordinary air. Nitrogen does not remove the need for pressure checks and does not prevent punctures, valve leaks or wheel-seal problems.
What is different about the fill?
Ordinary air is already mostly nitrogen. A nitrogen-filling service increases that proportion and can reduce moisture in the supplied gas. Gas permeability and moisture can matter in particular applications, but they should not be converted into an unsupported promise of a fixed pressure-loss rate for every road tyre.
Both air and nitrogen respond to temperature. A nitrogen-filled tyre still changes pressure as it heats and cools. The gas choice does not change the vehicle's recommended pressure or the need to measure it appropriately.
Is it worth paying for?
Ask what the service includes and what benefit is supported for your use. Do not pay on the understanding that checks will no longer be needed. For most ordinary passenger-car use, a reliable gauge, correct settings and investigation of repeated loss are more useful priorities.
Specialist applications may have their own requirements. Those should not be assumed to establish a worthwhile benefit for every daily-driven car.
Can you top up with air?
Ordinary air can be used to correct the pressure in a nitrogen-filled tyre where inflation is otherwise appropriate. Do not leave a tyre underinflated simply because a nitrogen supply is unavailable. Follow the vehicle's cold-pressure guidance.
A visibly damaged or flat tyre needs assessment; the correct response is not always to inflate it and continue driving.
If a tyre keeps losing pressure
Record the loss and have the tyre, valve, bead and wheel checked. A different filling gas is not a repair. Likewise, a pressure warning should be investigated rather than dismissed because the tyre was recently filled with nitrogen.
The practical decision is straightforward: choose an appropriate fill, maintain the specified pressure and investigate a fault. Nitrogen should not become a reason to neglect any of those steps.