I did go all in and it's working out fine so far. But huge lesson to all: no matter how good the prices are, never invest all of it. Always leave yourself some USDT, at least 10% probably 20%.
Because the market, yes even in a bull market, will ebb and flow. That means it goes up and then down. Always. There are dips 2-3 times per month, whether in BTC or certainly altcoins.
The reason you want to always have some uninvested USDT/BUSD is because of these dips. Like yesterday and today's. It's basically a 50% hit to many people's portfolios, wiping out months of gains in 48 hours. BUT, and here is the big BUT: if you left yourself 15% of your money in USDT, you could have bought that dip. And many of these alts have already gone back up considerably and while you'd still likely be negative (basically anyone who started investing in 2021 probably is let's be honest), you'd be quite close to breaking even. Others have no choice but to sit back at the mercy of the market and wait, perhaps an entire month, before they get back to to break even. Others will chase risky trades, to try to get out of the hole.
This sort of taking profits, and leaving some USDT on hand at all times, and constantly expecting dips (check the charts, they always happen with regularity, even in bull markets) overall is the 100% the best strategy, with buy and hold the only other viable strategy to me. With this incremental, defensive strategy, you trade wild fluctuations in your portfolio. No homeruns, mostly singles and doubles. But also no disastrous errors. The up and down swings in your portfolio will be less severe, and I think in the end, your profits are better protected and your max profits, might even be more long-term than just trying to buy the bottom and sell the top.
TLDR; always leave 15-20% of your portfolio in USDT simply for the reason that dips happen at least twice per month. Being able to buy the dip allows you to get out of the hole quicker, is better for your emotions, and though there are too many possibilities to quantify, I think over time your profits will be just as good. The only other viable strategy in crypto is to just buy and hold. It's alot less work.