I'm going to basically agree with Kenny on this. Being a safety pilot as an IR student is a great way to build familiarity with the radio chatter, get an idea of what's going on (especially if the person you're safety piloting for explains what he/she is doing). So, from that perspective, I'd recommend it. During the first month or so I probably spent about 5 hours as safety pilot for a friend of mine as he was finishing up his IR. It was neat to follow along, and I certainly benefitted from watching him do approaches, etc... oh, and make mistakes.
I would highly caution you with using safety piloting as a means to build time. Not only is it possible that it will get rejected, you aren't getting that same learning experience about making a trip by watching someone else doing it that you are if you're doing it yourself.
Another thing I'd caution: as I am learning as a new IR pilot, being proficient in instrument flight is about a lot more than meeting your currency requirements. I would tend to believe that the person with an instrument rating who only occasionally uses it is likely to be more dangerous than the VFR pilot who knows instrumeny flight is a non-option. I'd encourage you to get the rating, but then you either need to keep up with it enough to be proficient, or else tell yourself that you're just not going to bother using it without a CFII in the plane in actuals. Also, make sure that your first time in actuals isn't after you get your rating and you're flying by yourself. Make your CFII take you up in actuals. If he's not comfortable with that, find a new CFII.
In terms of what people are looking for in a safety pilot - the main thing is just be the eyes to look out for things - ground, airplanes, etc. This is especially true when you're in busy airspace and likely to have other planes around. Be prepared to take the controls (and make sure the guy under the hood understands "my plane"), but I personally try to make sure to give the pilot enough notice to give vectors.
For approaches, it does help to follow along. This is both for your own learning, but also to know where he's supposed to be at. For example, I was safety piloting for a friend who was doing a VOR approach before I knew how to read approach plates. Well, at some point the trees started to get a little closer than I thought they should, so I asked him what altitude he was supposed to be descending to (not knowing). Turned out he was misreading the altimeter by 1000 ft. Oops. It would have been helpful to know how to tell that.