My advice was good if your goal is to maximize the long term profit impact of your gems.

Spending gems on stock doubling to upgrade decorations does increase your income, but they're heavily effected by diminishing returns. Every upgrade costs more, and gives you less of a relative benefit, than the previous upgrade. By the time you get a decoration up to about level 60, you'll be spending millions of stocks for its next upgrade but seeing extremely little change in revenue from the purchase.

The store multiplier effectively has zero diminishing returns. No matter what your current store level is, spending 300 gems on the multiplier there always has a noticeable impact on your revenue. You get huge bumps in value every time you upgrade the store to the next level (which is, what, four or five 300gem upgrades?). The Store Multiplier is the single most significant factor that governs how far you'll get in tournaments.

A gem is always a gem, its value is determined by how you use it. The value of a girl's level (her worth in stocks when you sell) is NOT fixed, every level a girl has is worth more stock than the previous level. When you "double shares" you're essentially multiplying 10gems*GirlValue. If your GirlValue is a relatively small number, you aren't getting much benefit from gems spent on them. There are stronger ways to use gems.

It might help to think about "double shares" in relation to time. If you start a company, play for an hour, and then sell you might get 1 million shares. You can spend 10 gems to double that, meaning you gained a measly 1 hour of benefit for 10 gems. If you do that for 8 hours a day, you will quickly get to the point that selling a company after 1 hour doesn't come close to giving you enough stock to upgrade anything. You'll also run out of gems, or spend a buttload of real money buying more of them.

Even if you set up a complex macro to play for you (so it automatically assigns girls, clicks/upgrades for an hour, sells the company for double shares, and then starts over) it would take decades of continuous "play" for that method to gain enough stocks to max everything out. You'd be getting almost nothing from all the gems you spend doubling shares.

Yesterday when I posted to this thread I had 51500 total level ups, which yields 881.83 e9 shares.
Right this second I have 53650 total level ups, which yields 1.68 e12 shares (I can double it to 3.36 e12).

Those are real numbers taken from my game.
I gained 2K levels over the course of a day, yielding another 800 e9 shares (without gems).

Let's pretend that you have unlimited gems at your disposal for stock doubling. How many years do you think it would take you to get 800 BILLION shares, which I did in one day, by quick-selling your company hundreds of thousands of times? More than a year, right? Probably lots of years.

How long would it take you to match the 4-5 TRILLION shares I expect to claim later today, or maybe tomorrow at the latest?

Not millions, that's chump change.
Not billions, that's just small bills.
TRILLIONS are needed to max out your decorations.

The math doesn't lie. Quick sales can feel good when you don't have many decos and nothing is expensive to upgrade. In the beginning of your CEO career it might make sense to double shares after playing for a couple of days, but in the long run you're dramatically slowing your progress if you continue spending most of your gems on double shares.

Yes, I will spend for double shares when I finally finish this run. Those 10 gems will be gaining me 2 months of dedicated work, they're unquestionably worth the cost.

I do not expect to ever spend another gem on stocks after this run, because the only decoration I won't have completely maxed out is the neon sign. It won't be worth stressing over that hyper-expensive sign when there's nothing else to spend stocks on

Once my decos are done, I will be putting every gem into the Store Multiplier until I'm able to hit level 15 in the tournament (I got to 12500 last weekend). That's the final major milestone in my book, because it maximizes the amount of extra stuff I get for free every week.

After I can hit 15 in tournaments gems become a toy, because there's no real benefit to increasing my income further. That's when I'll start using gems for additional outfits and other things that aren't guaranteed to help my income.