Well, there are two schools of thought there...
Some will say yes, because the larger the volume of water in your system, the easier the setup will be to maintain, not sure who's quote on here this is, but... "dilution is the solution to pollution!"
The other though is that by having the tank independent of your display, Your frag tank will not be introduced to any contaminents from your display. If you're new to the hobby though, chances are you won't be needing a frag tank for some time, you can simply grow some frags here and there in the display tank

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Andy
My 29 Gallon Reef! My DIY LED Lights Current Status - Camping - Back 7/21/08
I'm still around people, just on a sort of "summer-time leave"

Feel free to PM or e-mail me for any needs!