Certainly you should have contact with the conductor when you play in the orchestra, in any orchestra, and if for example the singer slows down, or the choir rushes, but the conductor signals to the orchestra to hold the tempo steady, then you hold that tempo. In some cases you may even exaggerate the beat so that the choir can hear it clearly and sing in time with it. I will do that if I hear the choir rushing and feel the conductor is trying to hold them back. It works too and the conductor usually throws me a thankful smile
However, there are other situations where if you rely solely on the conductor to time your notes, for example pizzicatos during arias where you may have complex rubato to follow or need to catch the downbeat following the singers fermata and so on, you will often not be together with the singers and you'll just stick out, in a bad way that nobody intends.
The conductor may try to show these details but conductors are.. less than perfect. So are we in interpreting their gestures.
I know some musicians who think that its ok that they're 'with the conductor' (even though they often aren't, but they'll insist they are anyhow) when their pizz. comes early, but the singers don't think so (the best part of their aria may have just been ruined thanks to the bassist who didn't listen to them come out of that fermata), the other musicians don't either (because they are listening and expect everyone else is too) and certainly the audience will not - they are listening primarily if not only to the singer and assume anything not fitting with that beautiful melody is out of place. And you know what? They're dead right!
Be a musician, not a machine. Watch the conductor, follow his intentions, but keep your ears open. This is so important in opera..