DAC placement

Does it make much of a difference where an outboard DAC is placed?--should it be on a separate shelf?

Im using my cd as a transport connected to an outboaed Benchmark DAC
I have found that isolating them from vibration etc is helpful, a seperate shelf would be good but not essential. I have my transport and DAC sitting on top of a roller bearing platform which is itself sitting on my Star Sound rack. They are seperated by a set of Ceraball isolation devices with a VPI Magic Brick sitting on top of them. Probably overkill but sounds good. I have found that most componets sound better with a weight on them, does not have to be exotic or expensive.
Make sure it is below the digital source so that the electrons can flow easily to the DAC. ;-)

Seriously, it does not matter. Make sure it has a clean power supply, it is well ventilated, and cabling is no longer than necessary. Vibration is unlikely to cause problems with the sound of modern equipment but may affect tubes. Excessive vibration may shorten life span. Keep high voltage cabling away from signal wires and keep the unit away from high RF/EMI sources like transformers.
I think that any sound isolation or coupling is going to be more important on the DAC than the transport because any effects will be stronger in the analog signal. I know that at my local Hi Fi store they had Nordost Sort Kones under the PS Audio DAC but not the matching transport. I'm convinced of the difference that Sort Kones made for my SACD player, but I would attribute it to the D/A portion of the player.
Since your Dac is not tubey, you dont have to worry. I usually place mine besides the cd player. It is recommended to Leave a gap between your DAC and pre. (if you have one)
Don't be fooled. It can be just just as important to isolate solid state as tubes. In the case of the PWT and PWD both benefit from isolation treatment. I use Herbie's Tenderfeet with 2 Supersonic Stabilizers on top of each component. For proper results of any isolation system one needs to first locate the component where it sounds beat on the shelf if you are using one. Then experiment with footer placement, then component top weights. Besides the Herbie's products, I've used VPI Bricks, MpIngo Discs, Vibrapods and Vibracones, and cones of various makes and sizes. Some components, like my ARC Ref3 don't seem to need footers as the stock feet are good enough. Also if you can move your component racks so they are situated on the middle side walls, you'll hear a difference too. Vibrating components smear the sound and you can loose the subtle sonic details that can really make your system sound outstanding. Don't forget raising your cables offs the floor as well. I've found that each of these tweaks can improve the fidelity of a system and when they are all added up the difference can be profound.
In my experience, isolating most components other than front end devices (turntables, CD transports and spinning disks) is unnecessary. As long as your input and output cables are snug and do not wiggle about (USB cables are notorious for this), you will be OK. It doesn't matter whether your gear is tube or SS, unless it's badly designed or cheaply made.

I have designed and installed audio data collection systems (including DACs and ADCs) that were installed into 4WD vans for travelling at speeds up to 70MPH over the desert (no roads). The suspension system for the equipment racks only damped vibration over 6gs. We tested up to 6gs with and without suspension - there was no recordable or audible difference as long as the cables were tight. Some of the gear used tubes (large power transmitter tubes).
Isonodes and Isopods under everything (including Isopods under my Silverline Preludes). The Isopods are so sticky they keep anything exactly where you put it...my DAC could withstand a pipe bomb. Be sure to note that too much isolation can cause components to become lonesome and depressed. Also, I play electric bass and guitar (frequently) through high quality "combo" amps (mostly)...there's your isolation paradox! My "boutique" little all tube guitar amp must be torturing itself, but since it sounds great I just look the other way.