I don't think I'd have the nerve to PM someone for a sub for sub...that's what the 100 threads are for XD I post in those...but...I think it's a bit bold to send a personal message...
I guess it's not the worst offense, but...it is a little brash.
Hm...I guess if it happened to me, I'd check out the comic, and if I was interested I might sub, but if not...I'd probably say "no thanks."
Reason being, I'm pretty busy and I'm subbed to a lot of comics already...and I'm terribly behind in reading them o_O (I do try to read everything I sub to.) It's hard to keep up (and I'm slow anyway...)