Question
Should I pinch these seedlings?

Answer
The Plant Biology website offers illustrated instructions on growing this Ipomoea, a close relative of the Heavenly Blue Morning Glory and night-scented Moonflower Vine:

'Once the young Morning Glory seedlings have emerged they should be thinned to be about 30 to 45 apart, and given a trellis to climb upon. Once morning glory starts to climb on the trellis, pinch back the growing tips to encourage branching growth.'

For further reference, behold the URL: www.plant-biology.com/Ipomoea-morning-glory.php

I'm guessing your Red Cypress Vine is Ipomoea quamoclit, also called the 'Hummingbird Vine' because its red flowers and tube shape makes it the perfect dinner for Hummingbirds.  Website Operation Rubythroat lists it as #3 on the list of 'Top Ten' to attract Archilochus colubris.  Here an image of Archilochus at a feeder that looks just like one of your flowers:

auth.gardenweb.com/members/jenny_in_se_pa

The author/photographer is a former chemist.

Here's a photo of a Hummingbird Vine sans Hummingbird:

http://images.search.yahoo.com/images/view?back=http%3A%2F%2Fimages.search.yahoo...

Look familiar?

Pinch?  Yes.

L.I.G.