If you sit down and do the numbers... Casting DP with 0 stacks of SW is greater dps than having to delay DP for 5 stacks of SW.
I'll give an example... lets assume DP ticks for 1K (with 0 SW stacks)... if you cast DP first it does a total of 8K damage in 24 sec.
Lets say you decide to cast 2 x full duration of MF to get to 5 SW stacks and then cast DP. This means DP doesnt land on your target for 6 seconds (assuming 0 haste for ease of this example) meaning you will only get 6 ticks of DP in the same 24sec time frame. Each tick is 10% harder so DP ticks for 1.1K but only manages 6.6K damage instead of 8K as illustrated above. You end up 1.4K out of pocket (so to speak).
At the end of the day your opening rotation will have an insignificant effect on your overall dps (assuming you cast SW:P with full debuffs up and 5 stacks of SW). If an encounter lasts over 5mins, what you do in the opening 20sec is hardly going to impact your overall dps in any meaningful way.
However if you are keen to output the absolute maximum, VT/MB/DP/MF/SW:P (assuming all debuffs are up, otherwise you delay casting) is the best way to go.