When I run official games, I have to have painted minis. It is one of the Catalyst Agent rules. ;)
We are allowed to proxy though.
In my case, I tend not to. I tend to pick 'Mechs that I'm going to run from 'Mechs that I have, and if I really want to run a 'Mech I don't have, I buy that 'Mech. Since most of my 'Mechs are from 3025, 3050, and 3055, you can guess which 'Mechs are most commonly on my tables. :)
In non-official games, I'm perfectly happy placing an unpainted mini on the table. We are playing with eight crusaders on the table. Great. Let me grab my six painted ones, and a couple unpainted ones. Here they are. :)
Now there are times when if I really really want to use a 'Mech and I don't have it, I will proxy. I will find a 'Mech that looks kinda like what I'm running and is the same class. I don't have a Devastator for instance, but if I were going to put one on the table, I would probably proxy it with an Emperor that I DO have. A heavy crabwalker with guns in the arms? I'll grab a Marauder mini. A light with a single gun in the right arm? I'll grab a Stinger. Stuff like that.
The one thing that does annoy me when I'm playing is when someone proxies and I call out the mini <Thunderbolt>, asking if it has moved or what it's mod is or declare I'm shooting at it and they look confused for a moment before telling me "oh...that's a <Crusader>." If you are proxying, it is YOUR responsibility to keep track of what you are playing. If somebody calls out your mini, don't look confused. Know what you are playing. The only reason I'm going to look at your sheet and police what you are doing is if I get the feeling you are cheating. Short of that, I don't care what the name on your sheet is. I'm firing on that big honking heavy bag of bolts <Thunderbolt> in front of me. I only care it is a <Crusader> when you start firing your missiles at me. :)
Of course, if you were playing a Crusader and didn't have a Crusader mini, I would reach onto my shelf and lend you a Crusader mini for the game. ;)
So yes, I'm more WSIWYG than not, but I WILL proxy if I have NO OTHER choice. But if you abuse the Proxying with Four Atlas minis representing a Wasp, a Wasp, an Atlas, and a Wasp, or anything like that, I will quickly make certain that the minis on the table are REPRESENTATIVE of what is on the table. >:(