Bottle dregs can be a mistery - a fun experiment but I wouldn't go that route for my first batch. Use a commercial culture to get more reliable results.
You can do both. Start with the Roselare/lambic culture and add sour dregs to the carboy later.
If you're going for more of a Flanders Red/Brown type of beer, go with Roselare, and transfer out of primary into secondary, then add dregs from other sours.
If you're going for more FUNK (lambic), start with a lambic blend from Wyeast/White labs, keep the beer in primary, and add sour dregs.
In either case, I wouldn't start tasting for at least 6 months (except for a final gravity reading).
Use the resource given above, its great! Also, Mike Tonsmire has some great info on homebrewing sour beers on his blog, TheMadFermentationist.com. He provides a short list of commercial beers worth harvesting here:http://www.themadfermentationist.com/2010/06/harvesting-sour-beer-bottle-dregs.html
Will this be an extract or all grain batch?