I have performed at fundraisers in which we received a fee, and others in which we donated our services.
For our band, It depends on the type of event, and the scale. Fundraisers for an individual or family in crisis, for or raising money for disaster relief are some I've participated in or would participate in for free. Or when the St. Patrick's Day Festival was rained out and lost $$$, we played for free at the "wake" to raise money to pay the bills. It was a social event, and supports an organization that hires us annually to play, and includes a lot of community members who support and hire our band.
I highly recommend sending a "letter of understanding" to the organization chairs, stating the full value of the service being donated. Type it up like a contract or invoice, then draw a line through and write: "fee waived". The dollar amount should be your highest typical fee, not discounted. Then write it off on your taxes. This helps the organization appreciate that the music and time being contributed has a substantial financial value.
Once we were asked to play in a concert series to raise funds to preserve an historic building (concert held at the building). This paid off by giving us chance to have access to the building as a venue for a concert.
Generally I would say that fancy events where catering is involved and guests pay substantial amounts to attend, have an entertainment budget and the performers should be paid their normal fee. The organizers sometimes don't respect you if you value yourself too cheaply.
I have actually thought of contacting organizers of major charity events to market our band - for cocktail hour entertainment etc. But haven't gotten that far. I think the roster is not a bad idea.