Declawing a cat isn't like giving it a manicure. It's an amputation. The human equivalent would be to surgically remove the tip of each of your fingers.
Animals cannot give consent. When humans cannot give consent (due to age, infirmity, or incapacity), the standard approach is to substitute in either a real or imagined agent to plead on their behalf. A useful common-sense heuristic is something like a "reasonable person" test. Would a reasonable person be expected to decline an offer to have his radial phalanges lopped off? What are the costs and benefits of extending this common-law consideration to brute animals?
Going whole hog is probably not practical. Animals, like it or not, are still food. But it's hard to deny the empirical observation that pets, at least on some margin, are a luxury good (that is to say that the wealthier people are, the greater the fraction of income is spent on the welfare of Fluffy and Mr. Bojangles). I have no doubt that compared to the harsh, flea-bitten lives of their ancestors, Paco and Skippy Squirrelbane lead absolutely charmed lives. Empirically, it seems at least plausible that a weakened version of standard human common law jurisprudence for pets is headed down the pike, and that if we are willing to view non-human creatures as having at least some moral agency, then the caretaker-ward relationship between homo sapiens and their companions might well move a step or two towards the euvoluntary end of the spectrum if we lend them a little extra dignity.
Please think twice before declawing your cats. If you're not willing to have your furniture shredded a little bit (or are unwilling to provide alternatives and a little discipline), perhaps a feline isn't the right companion for you. A lagomorph might be more your speed.