No expert, but I say as long as the plant is healthy and is getting enough light, then let the flower stalk mature.

I have not cut any flower stalks off of my plants... VFTs, Drosera, Sarracenia, etc yet and have not lost one yet. However, I did lose one flower stalk due to a bout of frost in early spring on my S. rubra ssp jonesii.

The thing of cutting the flower stalk is done to the Venus flytrap because it takes a lot of energy for the VFT to grow the flower & develope the seeds. That can effect plants that are in stress or not well established.

But the cape sundew are not effected by that. they will shoot flower stalks in my experience up to 18 inches with 30 to 40 flowers. Also, if you grow your sundew well, they can live up to 10 years, my experience also.

Another thing, Cape sundews can be grown without the need of an terrarium. And finally, Cape sundews are the weeds of the carnivorous plants.

You'll see that pretty soon.

Anyways... Good luck!

Yah, If you want seeds let it grow...
The flowers only last for like a day, but they are self-pollinating so after about a week or so they should start to swell up with seeds...
They'll turn black when they're ready...

You should get a ton of seeds from 3...

Good Luck With Them,
Eric