Putting a sticker on the front doesn't look as good as printing an envelope which is what most of the big companies do. I can see that for mass mailings of monthly statements or something, but not for sales and marketing to potential new customers.
I'm a small company, but the way I do things has most of our customers thinking we are a large organization cause we always do things neatly and we always take care of customer's needs.
This doesn't cost much more at all and it gets us more customers because they have more confidence in us versus some of our competitors that do things like putting stickers on envelopes.
But, we all have to do what we think works best...
I like to custom print envelopes when I send proposals and/or marketing material out to the occasional customer that doesn't do email, or that prefers something by mail.
Just want it to look very clean and professional.
And, when I send a personal letter out...I want it to look nice!