Are they safe? Yes if you get a set of hubcentric one's. I've been running wheel spacers on my BMW's for years. This includes a full fledge race car that see's speed of 150mph on a regular basis. Never had an issue with them as long as I used all the correct hardware and they were hubcentric to the wheel/hub.
However based on the fact that on the Jeep's I'm not running a $1500 set of rims I have to ask why would someone drop the $$ on spacers (they typcially run right around $200 for a set of 4) when you can simply buy a set of rims with the correct backspacing and not have to worry about running spacers. The only benefit I can see with running a set of spacers is if your running some special set of rims and need more clearance. Other than that I think they are a big waiste of money and you can just buy new rims for the same amount.
I was just wondering because of the stance of my XJ with the BW flares on, I hate how it look's, Was thinking that with wheel spacer's on, Putting the tire's out some might give it a better look, I found a set of 4 on E-gay for $ 119.00, They are 1 1/4 wide.