It's advised not to delete subscriptions. Doing so throws your records off. At any rate if you want to delete them then yes you should do so before deleting the user. You can leave it to expire, force it to expire by editing it in CBSubs > Subscriptions, or delete it from _cbsubs_subscriptions database table.

It's test data that I want to delete because they throw my records off by existing.

Ah, I see. You'll need to find the rows within _cbsubs_subscriptions database table and delete them manually. There's no delete function for subscriptions in backend except when doing so directly through user management.

This is something i meant to ask anyway, as If I want to manually add bonus months etc to subscribers how can I do this myself from the backend?

Normally best not to modify a subscription, but I suppose you could just extend the expiration date by a few months, which you can do by editing the subscription within CBSubs > Subscriptions and clicking the # under the ID column. Don't do this for auto recurring subscriptions though.

Is there any risk of displacing anything else by deleting these rows?

No, but you should make a backup of your database before making any changes. If you added the test subscriptions from backend then there won't be any payment or basket rows. If you tested from frontend you'll probably need to remove the basket and payment rows too, but you can do that from CBSubs > Baskets and CBSubs > Payments as both pages have delete buttons.