Are you talking about rhizome pruning or trimming of branches? I've never had to trim any bamboo, including Silver stripe, to "keep inshape."

my silverstripe looks good and is in the ground. I think it will be a few more years before it gets nice and big. though.

I did put it in the ground so we will see. It has two new shoots starting now and it hasn't been a fast growing bamboo.

Depends on what look are you going for, If you prefer to see the culms then yes you will have to prune the lower side branches occasionally. They get about 25ft tall, in their ideal climate, and yes eventually, the diameter of the clump does get quite wide in ratio to it's height- that's just it's normal growth habit- but it's nice to see in my opinion, because all those canes make awesome garden stakes and trellises for my veggies.

I have 2 stands of B. Multiplex Silverstripe in the ground and both are awesome! Both are also currently "black sooty mold" free and hopefully they stay that way. This is often a common problem with many of the multiplex varieties.

Here is a link that might be useful: My B. Multiplex Silverstripe