I think it's perfectly valid to be less aggressive in buying stocks today than a few years ago. A few years ago, the rational (albeit difficult) choice for someone with a long-term time horizon was to pile everything they had into equities. If you had a mortgage, pay the minimums and invest anything else. Forgo any excess consumption so you have more to invest. If you had a paid off house, refinance at low rates and invest it.
Nobody knows where the market will go in a month or a year, but it is clear that the expected return on buying stocks today is far lower than it was 5-6 years ago, even if you have a long-term time horizon. That doesn't mean that the market will crash; stock returns can be low without a crash ever happening, or at most a minor correction happening versus a crash like 2008-2009. I think people of our generation are biased into thinking significant market crashes are common events, because we grew up with the 2000-2002 crash and the 2008-2009 crash, where equities fell 50% in each case. But throughout history, crashes of that magnitude are relatively rare. My point being, while expected returns are clearly lower today than a few years ago, reducing the attractiveness of buying today, that does not mean that you can wait around expecting a crash. There may be a correction or crash or there may not be, and nobody knows.
So my advice is to be less aggressive, but not try to time the market. There's a fine line between those. What I mean is, don't use leverage to invest in today's market, take advantages of situations that offer an assured return over investing in stocks (i.e., pay off a 4% loan instead of investing in stocks, if you have any debt), and only invest money in stocks that you will not need for a long period of time.