It may be safe, but it isn't free. As with most other things, you pay a risk premium -- in this case, in the form of failure to qualify for capital gains tax treatment on the resulting gain, because you didn't exercise in time to hold the underlying stock for more than one year. Depending on the amount, this difference can be quite significant.
You do your own taxes so probably know all this already, but here's a simple example[1] anyway. Let's say you "risklessly" exercise options with a strike price of 100 and a FMV (tax-lawyer speak for fair market value) of 1000. You have immediate gain of 900 -- and because you didn't hold the shares for >1 year, all 900 is Ordinary Income, generally taxed at higher rates than capital gains. (Top federal OI rate is something like 39% last time I checked vs something like 17% for cap gains.) Assuming the OI rate is 39% and the CG rate is 17%, you pay tax of .39 * 900 = 351, for total post-tax cash of 900 - 351 = 549.
What if, instead, you had exercised speculatively, more than 1 year prior? You'd still have taxable gain on 900, but because you'd have held the stock for more than one year (and met some other qualifying factors I won't bother explaining here), your tax bill would be 17% -- meaning that you'd pay .17 * 900 = 153 in taxes, and keep cash of 900-153 = 847.
Not a huge difference when we're talking about gain in the hundreds, but adds up quickly if you're in line for tens or hundreds of thousands or more.
It's really just a question of how you evaluate different kinds of risks, and what you want to pay to hedge them. If you want to balance your tax bite against the risk that your company goes under or otherwise fails to deliver, you may still want to exercise early, but only partially.
[1] I'm eliding a few things and making some assumptions. Not legal advice, talk to a real tax attorney before making decisions, etc.