A 12v output won't blow up a 12v pedal.
Delivering too much current (1A or 1000mA) won't do any harm, as the pedal will only draw off what it needs, and reject the excess.
Digital pedals can be really fussy about power supplies, so a voltage sag isn't a great idea.
Sagging the voltage IS a very good idea with analogue fuzz pedals - you can get a really horrible, splattery fuzz sound if you're lucky.
It can also be useful if you've got a pedal with a high current drain (500mA or over) that is rated at 9v, for example the Zoom MS60B.
As with all power supplies, check the polarity - the convention (based largely on Boss pedals) is Centre negative, but it's worth checking that your pedal follows this.
Pedals can take more current than they are rated for, but NEVER more volts.
Ever.