Depends on whether you value time or money. Value time??? Then head directly to a park and park there. Value money? Park at DTD and spend a possible 90-120 mins getting to a park.
Disney doesn't intend for those who park at DTD to get to a theme park...so they don't run buses between them. Do they stop you from getting on a resort bus at DTD if you aren't staying at that resort? No.
But...here's why we tend to discourage people from parking at DTD and taking a bus to a park. If you want to get to a park early in the day (before 10am), you will be hard pressed to get a bus to a resort quickly. Let's say you want to go to MK...you would have to wait for a bus to the CR...could be a 30+ min wait. Then, the ride to CR...about 15+ mins, and then walk to MK..another 10 mins...so, best case scenario would be an hour. But, you could wait a lot longer for the bus to arrive.
Since there are no buses running between DTD and the theme parks, it really isn't a viable solution to park at DTD for park access.