Especially if you keep some money for yourself, it's a good deal.
It seems to me that either it it's better for SIAI to have the money, in which case it's a worse deal, or it's better for you to have it, in which case keeping all of it is the best deal. I don't see why keeping some of it would be optimal.
There is a third case that may occur: you don't sign up, nobody gets the money. From all perspectives this is a bad thing. If you pay yourself some money, you decrease the proportion of "no money" to add to the proportion of "SIAI gets money". Keeping 5% means the SIAI gets 95 dollars (matched) instead of 100 - but if it makes a significant dent in the chances of the SIAI getting 0 dollars then the expected outcome is higher for paying yourself than it is for sending it all to the SIAI.
That said, you could use this justification to overcome the inconveniences, and re-evaluate at the moment of donating to the SIAI, and decide to donate the full amount.
If I gave you $50 you hadn't planned on receiving, would you consider giving it to charity?
Here's your chance to find out.
Just in time for the Tallin-Evans matching fundraiser, ING Direct has started offering a free $50 cash sign-up bonus. I've personally used ING for 10 years and referred over 20 people to similar promotions of theirs in the past so I can confirm that this is legit.1
It's a simple, effective way to get started as an optimal philanthropist for free:
Full disclosure: I was an SIAI Visiting Fellow in 2010. I've also used ING Direct as a customer the past 10 years, but otherwise have no financial interest in them.
[1] This isn't one of those bogus "intro" deals where you have to make sure you cancel the service later on or risk getting charged fees. ING has no fees, no minimum balance requirements, no sleazy marketing emails, and consistently good savings rates. If you want to use them for their good service after signing up, great, if not, no worries. All you have to do to qualify for the $50 is make 3 small purchases you were planning to make anyway with their new debit card. I know it's a trivial inconvenience, but I think it's worth it to be able to donate up to $100 to charity without actually spending any money.
[2] Profit denominated in warm fuzzies, karma, and post-Singularity catgirls.