I keep ONLY "Great Model Railroads" and "Model Railroad Planning" (I have every edition of each)
Yeah, I only keep those and any issue of MR/RMC that might have a article that I'm sure I'll go back to someday. They stack up really fast and every few months, I'll go through the stacks in the book case under the layout that hold the magazines (I have hundreds of 1:1 scale RR magazines and those for other subjects, including softbound books that I intend to keep) and cull out those I know I won't likely need.
I usually put a stack in the containers with the stuff I take to sell at model train shows, and I'll give a few to any decent-acting kid looking at my sale table who wants them (usually, they'll want them). Kids who tear stuff up or are 'handsy' get nothing other than what their parents pay for. I'll often drop off a few at the break tables where I work, as there are hundreds of people in my building and they always vanish right away. In the past, I've known co-workers who had kids who liked trains, and those unwanted magazines usually went to them.
Magazines, I think are like cars; they're valuable if they're really current or really old, and generally not so much between those two points. The only model train magazines I've ever bought were a few Great Model RR or Planning issues, MR's 50th anniversary issue (which I'd lost my copy in a move) and not long ago, MR's very first issue, in a bound volume with the whole volume one from the first year.
Really, older model train mags don't hold much value for people that I'd ever seen other than layout articles that hold personal interest for someone, or for plans. Many of those older articles are really outdated now and aren't much use to most people, that I've ever seen, anyway...