I'd just leave them tied in if it were me. hen it is as tall as you want then take out the growing tips.

Thanks Seaburn.

If the main object is just to cover the pergola, and the fruit is incidental, I would let it grow where it wants to, until the pergola is covered. Occasional tying with soft twine or the stretchy plastic tape may be needed, especially if there is nothing skinny enough for the tendrils to take hold.
If quality fruit is also important, then things become a little more complex.

What is “the stretchy plastic tape”, please? Is it a garden product?
As to priority over fruit or coverage, both would be nice!

The tape is pretty commonly available in garden centers her in the States, where it is usually sold as "garden tie". It comes in 1/2 inch and 1 inch widths, and its main advantage is that it stretches as the stem it's tied to grows.
As for the fruit, different kinds of grapes need different pruning techniques for best fruit production. Find out if the variety you have prefers "cane pruning" or "spur pruning, and you can go from there.
With both pruning types, though, the first act is to let the vine grow where it wants the first summer, and, the following winter, prune everything that grew that summer back to two-bud stubs.

Thank you Tugbrethil.
I will have to look for the tape, it sounds useful.
Thank you too for the information on pruning the vine. It was planted in Summer/Autumn 2020, though it was a mature vine when I bought it. It came with loads of bunches of grapes, which the birds promptly ate….. This year, I have little net bags ready to cover the grapes, so that we manage to get some too.