It's not wise to do so unless you will need the money in <10 years. The market is not meaningfully overvalued in terms of the ERP - and rates are only headed lower barring a sudden breakout in the CPI. So while volatility, selloffs etc. are certainly possible, it's not likely that the market will tank lower and stay at those lower levels forever. If SPX did drop 30% you'd be getting a 6% earnings yield, very attractive with bonds and cash at zero. With a long term view you should be hoping for a crash, since your contribs and reinvested dividends will be yielding so much more.
Another good long-term option is to diversify to international equities which might be more favorably valued, especially as the USD is quite strong - however historically these have sold off right along with the U.S. when we've hit a major bear.
If you will be needing the money fairly soon then you could take some risk off the table, and invest in blue chip rental real estate (even if discount rates rise tanking the property's value, your rental income will stay the same so long as the local economy holds up, the neighborhood stays good, etc), or just raise your cash balance - IB is paying 108bps on cash now which compares quite favorably with <150bps on 10 year bonds.
