I'm curious if there is any prevailing etiquette for choosing/writing a roster in a friendly pick up game?

I've started playing again recently and have been trying out various local gaming groups at stores. After asking for or being asked for a game and agreeing on a points value, one of two things tend to happen. In some cases, the other player has a roster more or less ready, which they might tweak up or down in points to match.

Other times, though, it seems that the other player sits down and hand-crafts a list designed to hose my army. In an extreme case a guy stopped and asked me how many of such-and-such I had, while writing out his roster.

Personally, I prefer to write my rosters in advance, which means they are not optimized against a particular opponent army, and feel that an opponent writing a custom anti-roster is taking advantage of me and, frankly, being kind of a weenie.

So...how common is it for players to write their rosters once they know their opponent's army and point value? Should I just follow suit? Is it unreasonable to ask upfront for an "all-comers" roster?