I have done both the look through the bore or a laser bore sighter.
I have found the laser bore sighter to be very fast to get on paper and then fine tune after I get to the range.
In my garage, I will take the bore sighter and insert into the bore and turn it on. I look through the scope and adjust the cross hairs to be coincident with the laser spot and call it good.
I remove the boresighter and reinsert and verify.
I don't need a target anywhere and can use the garage door of my neighbor across the street. I don't fret about the distance as I know I am going to be on paper at 25 yards.
Usually works best in early morning or late afternoon.
I have used it for rifles with and without flash hiders and with the proper washer, for all my handguns.
The only downside is the barrels must be clean and you need a little silicone grease on the o-rings to make sure they don't roll off.
The green laser is a little easier to see than the red one. It is also easier to distinguish if you are bore sighting a red dot sight like Eotech.