It’s up to you. If you care about quality or water resistance, do it.
I have attempted and successfully resealed my iPhone X, the experience should be comparable to iPhone 7, with ways to test seal quality included as well.
1: If you replaced the screen, don’t bother with water resistance anyway. You cannot be sure that the replacement screen has the screen bezel correctly bonded to the glass so it is sealed properly. The same cannot be said either for top speaker mesh if you touched that part. If you have replaced the internal components like battery or rear camera, you can have a try.
2: Use IPA wipe to wipe the contact surface of the casing and the display, make sure it is ABSOLUTELY clean. Inspect for physical deformation if the phone has been dropped. Deformation will result in a bad seal.
3: Clean the adhesive pack so that there are minimal dust or adhesive residue around the edge. Residue will collect on the side of the casing, then when the screen is closed, get scraped down onto the adhesion surface, bad seal again.
4: Apply the adhesive as shown in the iFixit tutorial video, don’t peel any of the front liner off yet.
5: Extra step. Use IPA wipe to wipe AGAIN to away excessive goo off of the inner side of the casing, so they don’t get scraped down with the display. Don’t use too much IPA, too much IPA will seep down towards the adhesive and potentially dissolve it. This step is actually needed because the repair environment and adhesive quality is not as good as the original factory or authorized service center.
6: Reassemble the phone, double check for dust along the edges, peel off the final liners just before you close up.
7: Use some heavy object to press down the phone to activate the adhesive. Use a little bit of heat if you are concerned about seal quality.
8: Vacuum test. Download any barometer app from the App Store, it will show the internal absolute air pressure. To test for a good seal, first squeeze the phone real hard and hold it, wait for air to go out through the vent then block the lower baro vent so air doesn’t come in or out, then release the squeeze. If there is a pressure drop and it doesn’t leak back, you have at least completely sealed up the phone. The test doesn’t work for iPhone Xs, Xs Max due to barometric sensor is no longer inside the case.
If there are any dust trapped inside the contact area, the phone may still be sealed but with compromised pressure rating. The adhesive strip is very narrow and the effective adhesion area can be even narrower if there is something trapped in between. Water pressure can tear it through so if you are not sure it’s absolutely clean before closing up, consider the phone as splashproof only even if the vacuum test passes.