One of the questions I get often--usually from young people still living at home or others who have to practice in secret--is if dedicating time to do volunteer work counts as a way of dedication in the same way a sacrifice might. It doesn't, but that does not mean volunteer work is not important or does not have a place in Hellenismos!

The traditional manner of prayer is accompanied by physical sacrifice and is performed in a ritual context. A sacrifice to the Gods is a way of bonding, of kharis. It's a way of showing our devotion to the Gods and bringing Them, actively, into our homes and lives. It's a way of acknowledging Their greatness and recognizing our loyalty to Them.

Traditionally, dedicating activity is not a way to honour the Gods. After all, it does not relate to Them directly, does not strengthen our bond with Them and They get nothing out of it. The energy is directed at the people you are helping, not upwards to the Gods. A small portion of that energy might 'bleed' up to Them but it's not going to establish kharis in the same way as sacrifice does. But it does relate to Hellenismos.

Hellenismos is a religion of Gods and ethics. Both matter and they strengthen each other. We are called by the Theoi to practice arete, the act of living up to one's full potential. The term arete was applied to anything and anyone superior. It is linked to knowledge and wisdom as well as physical beauty. It could even be applied to an exceptionally well crafted vase, the person who made it or even the seller, who sold it for more than it was worth. Needless to say it is also applied to those who live an ethical life.

Living up to arête is not easy: it challenges up to be our best mentally, physically, and spiritually. It means taking control of our life, to become an active participant in it. To place blame only on yourself when things go wrong, and to keep trying to reach your goals, no matter what setbacks you suffer. Arête should become a way of life, and in that way of life, an ethical framework is essential. Ethics give you the tools to create internal order and consistent action. Both are necessary for happiness. Ethics will remove doubt, fears and regrets from your life, as you know exactly what you should and should not do to become the best you can be. 
A part of becoming that very best version of yourself is to help those who need help. That could be your friend who is struggling through a rough period, donating money to good causes (which is why Pandora's Kharis was founded), or donating your time and energy to those who need it. There is even a Delphic Maxim that adresses it:

"Share the load of the unfortunate (Ατυχουντι συναχθου)"

I read this as: give what you have to help them through, be it time, energy, skills, food and drink, clothes  or anything else. I apply it to the refugee crisis currently taking place and I apply it to simpeler things like always giving money to those trying to raise it for good causes. It means having an open outlook on life and realizing that you, most likely, are a lot better off than many people around you. That is certainly true in my case.

So no, volunteering your time is not a sacrifice to the Theoi as such but it makes you a better person to honour Them. So it matters and I feel we should all do something--anything we can. Practice Arete every day and kharis will come to you.