ip firewall nat rule where you redirecting web traffic to web proxy under the Extra tab you can put time when this rule be active, you can put in your case 8-5pm monday-friday, so the rule will be always in your config but it will be active only for the time you specified, when it is inactive your web traffic will pass directly to the internet without any checking by web proxy
Though I had also tried using scripts and schedulers as explained here (http://forum.mikrotik.com/t/ip-web-proxy-allowing-sites-to-get-unblocked-for-certain-tim/52308/1) which also worked quite well. I prefer your option though.
I think the main difference with the two solutions is that yours allows me to disable or enable the entire proxy server, which is what I want, while the other one allows you to disable and enable specific rules on the proxy server.