Shares of advertising technology company The Trade Desk (TTD 0.06%) have soared this year. Indeed, the best description of the company's year-to-date stock chart is up and to the right. In total, the growth stock has approximately doubled during this period. With such strong gains in the rearview mirror, is it time for investors to sell and make their big win permanent?

When I asked this same question in early June, I argued that shares were worth holding onto despite their 66% year-to-date rise. But with the stock already gaining more than 21% from those levels, it's time to revisit this question.

Be mindful of risk

While it feels great to have a big winner in your portfolio, investors should be sure to remain objective. There's no way around it: a stock becomes less attractive when its price soars higher in a short period of time without any new information suggesting shares are worth materially more than they were at the start of that period. In other words, the risks to owning a stock increase when the valuation becomes more speculative.

Regarding The Trade Desk stock, the difference between where it is trading now and where it was just a few months ago is staggering. Its price-to-sales ratio has increased from 19 three months ago to about 28. Keep in mind that this is The Trade Desk's price-to-sales ratio, not its price-to-earnings ratio. Further, while The Trade Desk is growing rapidly, it's arguably not growing fast enough to justify a price-to-sales ratio of 28. The company's first-quarter sales rose 21% year over year to $383 million. 

While there are many reasons to expect The Trade Desk's strong business momentum to persist, the stock's valuation seems to be already pricing in market share gains for years to come.What may not be priced in at this valuation are the risks to competition intensifying from competitors such as Alphabet's (NASDAQ: GOOG)(NASDAQ: GOOGL) demand-side platform DV 360, the technological risks associated with operating in a rapidly evolving industry, potential long-term headwinds to The Trade Desk's lucrative take rate, or the risk of valuation multiple compression if investors lose some of their appetite for paying speculative prices for growth stocks.

All of this is not to criticize The Trade Desk but to remind investors that nothing is wrong with admitting that the risk of owning the stock today is much higher than a few months ago.

Consider your alternatives

So, what should investors do with the stock? While the underlying business is firing on all cylinders, it might make sense for investors to sell some shares at this price and take some risk off the table. This is especially true for investors with some other good ideas on their watchlists. Given the tech-heavy Nasdaq 100's 44% year-to-date gain, it makes sense for investors who are heavy in tech and growth stocks with high valuations to consider bringing more balance to their portfolio with more reasonably valued investments.

Put bluntly, it might be wise for investors to sell some of their shares of The Trade Desk in light of the stock's enormous run-up this year -- especially if you have some good alternatives for that capital.