To be or not to be, that is the question.
I'm wrestling with that question also now. I'll give it a try.
First, are markets high?
Even if markets are high, easy answer is to guide decisions on the long term outlook of your holdings. So, keep them unless overvalued++. Here, it depends where your trigger to sell is.
The harder answer is to lower your trigger to sell (ie more à la Graham). In general, a lower trigger definitely helps to avoid market collapses but 1- significant deviation to the norm may occur for long periods and 2- sometimes markets can remain irrational longer than you can remain solvent (Keynes) and then, you may miss out on compounding for some time. Maybe the worst scenario is to change your mind at the bottom or the top (a certain CDN company comes to mind here).
It really depends on your criteria.
In my own anecdotal and personal situation (ie not worth very much), I don't find anything exciting now but my universe is limited. Also, AFTER this evaluation of opportunity set, I find many reasons pointing to high valuations. Many of my premises though are influenced (biased?) by the fact by I hate debt and there is a LOT of that around these days. Allergy to high cash positions may also influence your decision process.
What is challenging is that you may have to wait for a very long time before validating your capacity to "manage" cycles. If you do try, my take is that you have to have very high confidence in yourself and be ready to be disappointed. Live and learn, some say. I personally think that is worth the try. Value investing is about buying low ans selling high, isn't it? Good luck.