Not true always, sometimes even when Bitcoin gets dips the altcoins that are pumping at that day won't get affected by the dip to huge levels. I have personally traded altcoins during the huge dips of the market and those altcoins which were pumping to high levels weren't affected during the dips.
I believe that a trader should always follow a good trading plan with his/her own strategy. One's own strategy is way better than someone else's strategy and when it's combined with a good trading plan then the chance of having profitable trades get higher.
Yes agree not all coins are affected by Bitcoin but most of the major coins are affected for every Bitcoin price action that is why there is a chart in tradingview related to BTC.D meaning dominance as you can see it is currently around 50% above meaning it still dominates most of the altcoins including memes.
That is why when you trade always analyze the BTC price action first because it can increase your validation and probability that the position you just placed has a high chance of winning that is why there is a bias included in the list above to increase your chance to win a trade.
@OP I just want to share this. Learn how to read the order book and liquidation map because it can also help where to entry and where to exit always be careful of stop-loss hunting other traders keep monitoring the market I already experienced this because my stop-loss is always hit by stop-loss hunters so beware you need to analyze price in high time frame because low time frame is usually manipulated by stop-loss hunters.