1. I normally SLOP if it puts me closer to any desired alternate airport, or to get me out of any wake vortex from a preceding aircraft (admittedly rare).
OTOH, I would consider no SLOP if it puts me significantly closer to a published track.
2. Probably a good idea anyway but I take the point that the 15nm offset may put you closer to another random route. TCAS is your friend!
Bear in mind that some random routes are quite busy, as they may be published tracks which have only just expired, or consist partly of a published track with a random segment at some point.