To be safe, you should never show ads if your domain name is close to an existing trademark. Especially if it's a reasonably valuable name that is worthwhile going after, whether generic or brandable.
If you own apples.com and a Mac ad showed up in the ad feed, you'd significantly hurt your defense in the UDRP process if Apple submitted screenshots of it. The panel lawyers are notoriously inconsistent and this would just give ammo for them to approve the transfer. Even if your registration was earlier than the date when the trademark was granted.
Having said that, there are attorneys like John Berryhill that could still successfully defend the UDRP. But if we're talking about a defensive posture then ad feeds should be watched closely or not used at all.