Give up work whenever you feel like it, or if you want to continue, cut back to one day or two half days if you can. With my first I was working on my feet too, I cut down gradually and stopped completely at 36 weeks - at that point I think I was doing about two or 5 hour days - if it is the nature of your work you could offer to be on call for occasional staffing emergencies if you wanted to. As it's your first, the time you have for yourself before bub arrives is very valuable and you will never have the opportunity again..plus there are plenty of things to be doing in preparation - it certainly isn't 'slacking off', besides what's wrong with 'slacking off' occasionally!
People have an opinion on EVERYTHING when you are pg...natural labour vs c section, drugs vs no drugs,breast feeding vs formula, to work or not to work...everyone is different so just do what is right for you.