Not necessary, anymore, no. I'd consider the modern-day intention behind saying something seems to be "I noticed you sneezed, I hope you're ok and not coming down with anything".

I could see "ducks" generating hostility and/or confusion, if it's obvious that you're using this in place of "Bless You / Gesundheit". This is more down to just people being weird, though; they also occasionally get upset or confused by me saying "Happy Hanukkah!" at the appropriate time of year.

My personal opinion is to not really care unless I am, say, at work and suspect my boss or co-workers might be offended enough to result in an HR issue, but it's probably worth being aware of.