Hi, welcome to GoY, no it isn't necessary, they naturally branch, and grow to about 9" tall, Derek.

Gosh - are your pansies on steroids, Derek? Mine never get that tall!

I do tend to nip out to keep them flowering and to stop them from getting straggly, but 9" tall - not for me as I would say they naturally flop when getting to straggly!

Hi, Viola x wittrockiana cultivars { pansy} are derived from cross-breeding V altaica, Vcornuta, Vlutea, and V tricolor, there are quite a few ' series', each with a number of varieties, apart from Avalanche Series, which are intended for hanging baskets and containers, and
produce flowers in winter and spring, and only grow to
2 1/2", the rest grow to between 6 and 9", so a lot depends on which 'series' you're growing, Derek.