When do you add spices to homemade butter

It's certainly safe. Butter, Oil, whatever, it's more about taste than anything else. Some people even push lard and butter as the "safe" options, because they require less processing than oil, and much less than margarine.

One thing which you should be aware of in regards to butter vs oil: butter has a significant amount of water in it, so you shouldn't substitute 1-to-1, or you will have a stronger flavour (and more fat) than perhaps you want.

If you're accustomed to salted butter, adding some salt will help ease the transition. If the flavour of olive oil is off-putting, there are plenty of more neutral flavoured oils that have a similar fat composition, or you can simply avoid extra virgin olive oil*.

*extra virgin olive oil in the U.S is largely a scam. In most of the world (countries that adhere to the IOC standard) this signifies "First Cold Press" olive oil. In the US it signifies "lamp grade olive oil with green food coloring". There are no restrictions on the use of terms like "extra virgin" in the US, so you can't guarantee that's what you're really getting just because it's on the label.

I have never added butter to sponge sheets when making swiss rolls, jelly rolls, or anything similar in the past. It's just straight up bread flour, sugar, and eggs.

I can only think of three possible reasons why butter might be added:

1. To improve the flavour;

2. To keep the sheets moist and help prevent overbaking, as any fat would be used in cake batter; however, this is generally not a major concern in cake rolls; or

3. To help prevent it from sticking to the sheet (don't bother; if you have this problem, use a silicone mat).

Reasons #2 and #3 aren't very convincing to me, so I would only bother adding butter if you prefer the taste of swiss rolls with butter added.

I don't think butter would help with rolling at all, in fact it might make rolling more difficult on account of making the sheets more crumbly and less able to hold together under pressure.