You should file an 83(b) election when you are issued shares with vesting (which is the standard). If you don’t file the 83(b) election, the IRS will consider each vesting milestone a taxable event.
For example, let’s say that you are sold and issued 1,000,000 shares at $0.01 per share at the time of incorporation with the standard 4 year vesting and 25% vesting after year one. The default rule with vesting shares is that at year one (when 250,000, or 25%, of your shares vest), that will be a taxable event. Let’s also assume that after one year, the shares are worth $1 per share. The taxable amount is the increase in the fair market value at that vesting milestone. Therefore, after celebrating the vesting of 250,000, you will then come to a quick realization that the IRS just attributed $0.99 of income per share[i.e. $1 minus $0.01] or $249,750.
The 83(b) election must be filed with the IRS no later than 30 days after the date of issuance.