Just because your brokerage firm is offering free trades, it doesn't mean you should wager your nest egg on your trading capabilities.

This week, J.P. Morgan Chase debuted You Invest, a brokerage service that gives clients access to free or discounted trades.

Individuals who already use the company's mobile banking app or website are entitled to 100 commission-free stock or exchange-traded fund trades in the first year. Users who exceed that amount and execute their transactions online will be charged $2.95 per trade — which is lower than the $6.95 per trade TD Ameritrade and E-Trade assess.

You Invest clients also pay no transaction fees for mutual funds they trade online.

Vanguard announced a commission-free trading platform for about 1,800 ETFs, on Aug. 21, and Fidelity and Charles Schwab offer commission-free ETF programs, as well as stock trades for $4.95.

Yet commissions are only one portion of the expenses involved in trading: Mutual funds and ETFs themselves have fees. And brokerage accounts may be subject to additional costs, including transfer and service fees.

"I'm not sure you should be doing 100 trades in the first year or in any year," said David Mendels, director of financial planning at Creative Financial Concepts in New York.

"Taking this opportunity to become an active day trader is almost universally a mistake," he said. "Maybe someone out there can pull it off, but I haven't met them."

Here's why you should proceed with caution.