I much prefer to design type as much as any symbology in a logo. Therefore, although type may be based upon some particular typeface, it's generally not a straight out "font".
So, if asked "what font did you use", I don't hide the name of the typeface, I tell they straight up. I will, however, explain that I used XXXX as a general basis but have altered the letterforms to better solidify the overall brand design and although similar to XXXXXX, there is no font which will match the specific glyphs in the logo.
If the client indicates they want to "match the font", I may express my opinion that doing so is unwarranted and that the brand/logo shouldn't be seen as dictating the overall type choices for any design. But that is just opinion after all.
Clients are going to mangle, degrade, alter, bastardize, or otherwise ruin your logo if they ask this question. They will. No way to avoid it. Perhaps not right away, but as time passes they will slowly change things to better suit that piece... or they won't notice when some other designer alters the brand for some specific usage. Asking this question immediately indicates they have some notion about how branding/logos should be implemented. Your opinion will most likely not sway them a great deal. Any attempt to dissuade them is most probably going to be seen as adversarial more than anything. So tell them the font right away, then explain your position on collateral usage.
Ultimately, for some client requests, all you can do is express an opinion, then give them what they are asking for. You don't really do yourself any favors by coveting such information. It's better to appear open and willing to comply with clients. By all means express your opinion, but don't be adamant about it. It's their branding, if they want to ruin it, that is their choice.