If my underlying thesis is still in tact for the company, and in the case of Alphabet it certainly is for me, I continue to accumulate and hold.
The only time you will ever see me selling off is if allocations get a bit too high for my liking. For example, if my 5% position in Google grew to 10%, I might trim back a bit. This is a risk-tolerance kind of thing and just downside mitigation due to over exposure to a single company.
But outside of that, investors are best just buying strong companies and holding them for the long-term. As Buffett once said, don't water your weeds and trim your flowers.
Both of these stocks could pull back, no doubt, but you don't need to be worried about short-term pullbacks. Focus more on the long-term thesis.