This might be shallow of me but if you have to resort to proxy's (unless testing a units use) then don't even bother with this hobby the hobby is expensive and if the person you are playing has to fork out all that money time and effort into making his army you should in turn do the same. If you must complain with "But i don't have the money" then don't even start as i said its expensive this hobby and it is a luxury to have.
To me i will only class a model as a proxy
if it does not fit the profile in the rules and the size is unreasonable (being more then 20% bigger or smaller then the model this is to prevent unfair advantage)
Anything else is a conversion in my eyes.