I think these are three separate, albeit related things --

1. Whether or not you feel like you can 'let go' in therapy as you'd like to,

2. Whether your T can actually handle it when you do and,

3. Whether you trust that your T can handle it.

I think to figure out if it'll be cathartic or therapeutic for you is something only you can decide -- we're all different and what may be helpful for someone may not be so for others.

But, I think the key is that a) you want to be able to do it and, b) you don't trust that your T can handle it.

I think judging yourself and stopping yourself from doing something that you want to do (which is not physically harmful to the T) isn't perhaps very helpful for you -- even if it turns out that letting go was not ultimately helpful to you, stopping yourself from doing it may perhaps always leave you feeling like something is lacking and that your trust in your T is not complete in a rather fundamental way.