Most books I've read about the Goetia tell you to give the offering once the demon has brought you the result you asked him/her for. But what about giving the offering in advance at the moment of evocation? I've read a couple times online that this could be a way of showing the demon that you trust him/her. What are your thoughts? Have you tried this method before?
In my opinion, offerings are not important. Some rituals work without offerings, and some rituals doesn't work with offerings. Logically, they're not a key element for successful magick. They help the practitioner psychologically, to have faith and believe the deal is done. That allows the magician to easily forget about the ritual and let it go. From a traditional point of view, offerings are a must. Giving offerings in advance - the best you can give - would be better because it shows faith and trust. Especially when it's the first time you work with the spirit. Later on, you can give offerings after the manifestation of your desire, plus some extra "thanksgiving" gifts.
Most important is to have a strong relationship or spiritual connection with the spirit or deity. With or without offerings. If you have a bad or weak relationship, offerings won't help much, no matter when you give those offerings or the quality of those offerings. Showing "trust" in a weak relationship would be a lie or a sign of desperation instead of strength. work to have a good personal relationship with the spirit, the rest will be easy and you won't have to worry about which is the best way to do it.
Do you trust him/her? If you do, why? You really shouldn't be trusting any entities that you don't have a lot of personal experience with. Even there, I wouldn't exactly say "trust" is the right word. Also consider, if you do not fully, truly trust that entity, I wouldn't recommend starting your relationship by essentially lying. Know what I mean?