For bulk organic matter, issues of exchangeability and water absorption will have to be examined on a case-by-case basis; however, for alpha cellulose, it is much more straightforward.
We have used a powdered Sigma cellulose (C1875-100G) in Utah (where it's pretty dry) and have seen little effect between samples that were vacuumed for multiple days and samples that were loaded into silver capsules straightaway and run. But it is a good practice to vacuum the samples to be sure (don't vacuum your benzoic acid!). I would not use sugar if cellulose is available, as it is even more hygroscopic (especially if ground) and is extremely difficult to dry.
As I'm sure you've read, the CO tank should only be used to monitor the instrument performance, not a reference for isotope ratios - this is why it is important to use at least two reference materials covering the expected isotopic range of your samples. Though the IAEA benzoic acids are not ideal, they have a very good pedigree to the VSMOW scale and are quite widely used, and have the advantage of flexibility in sample mass over the reference waters in silver tubes.
Though there are varying reported oxygen isotope ratio values for IAEA-CH-3, it may be useful for you to help verify your methods and calibrate your working standards. The original IAEA TECDOC-825 lists +31.85‰ using nickel pyrolysis and mercuric chloride methods. Posts in the archive give delta values of +31.5‰, +32.0‰, and +32.7‰. The reference below (Evans, et. al.) gives +32.52‰; we get values a bit more positive using TC/EA at +33.2‰. Though there may not be an officially recognized value, hopefully these can assure you that you're on the right track.
Best of luck!
Reference: Evans, M. N., et al. "Correction algorithm for online continuous flow δ13C and δ18O carbonate and cellulose stable isotope analyses." Geochemistry, Geophysics, Geosystems (2015).