Answer:
How badly do you want a response and how much are you willing to pay for it?

BREs (Business Reply Envelopes) are universally used in magazine subscription offers and many other customer acquisition efforts. Remember, you are not automatically paying upwards of $.35 for each BRE you send out - ONLY for the responses you get back! Since that is likely a small portion of your mail quantity, a cost/benefit analysis will often say to use a BRE and make it as easy as possible for your prospects to respond. One less decision should translate to a better response. Besides, you run the risk of alienating folks (and looking cheap) by not the paying postage and forcing respondents to seek out a stamp.

CREs (Courtesy Reply Envelopes) have valid uses as well. Here, you provide the reply envelope but not the postage to send it back. CREs are ubiquitous in billing efforts and have the advantage of being less - junk mail-like in the eyes of the customer. But remember, your offer needs to be solid enough to justify the added work of finding and licking a stamp before sending it back.

It is rarely a good idea to omit a reply envelope altogether in your promotional mailing. The production cost is minimal, after all. When this does happen, it is most often the result of a mail house mix-up rather than marketing strategy.

Regardless which type of envelope you choose, make sure it is machine insertable and that the customer does not have to fold the order form before returning it. Of course, a customer might also respond via a phone call (800# or no), BRC (Business Reply Card) or over the Internet. How do these fit into the mix? Ah, thats a question better saved for another time...