Well, if using 8-bit had ever been an issue for *actual mail transmission* -- I'd have heard about it by now (although I now think there might have been one such case, just not explained well enough by the user).
Other than that, assuming 8-bit clean SMTP has been just fine all this time.
But there could be a bug in Zoho's DKIM code

---
I have DKIM signing enabled on the aqua-mail.com mail server, which sends this forum's notification emails. This is postfix + opendkim.
Just tried sending a test message from that machine using command line "mail", with Russian characters (also requiring 8 bits).
According to Gmail, the DKIM signature was just fine, even though this message, too, was sent as 8-bit text without quoted-printable or base64.
Received: from mail.aqua-mail.com (aqua-mail.com. [176.58.105.125])
by mx.google.com with ESMTPS id j199si1868370wmj.84.2016.03.29.15.38.03
for <...@gmail.com>
(version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
Tue, 29 Mar 2016 15:38:03 -0700 (PDT)
Received-SPF: pass (google.com: domain of ...@aqua-mail.com designates 176.58.105.125 as permitted sender) client-ip=176.58.105.125;
Authentication-Results: mx.google.com;
dkim=pass header.i=@aqua-mail.com;
spf=pass (google.com: domain of ...@aqua-mail.com designates 176.58.105.125 as permitted sender) smtp.mailfrom=...@aqua-mail.com
Received: by mail.aqua-mail.com (Postfix, from userid 0)
id 13534100BE; Wed, 30 Mar 2016 01:38:03 +0300 (MSK)
DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=aqua-mail.com;
s=outgoing; t=1459291083;
bh=meOQOQU/44CzEVWy0So3PuNHfUYIXSGb0R1xhHLOr1E=;
h=Date:To:Subject:From:From;
b=mg4n6p2WpoUupQb4TGM7bI1OR7j3EInRYo+/qJu0Bwsrzs1IWgZ4LNh7lCpjLQoNl
DtBfJLf4M6agTabAzXsty1xcs/BcOMkpQq6qKheH19MVPOHSQW+I/hCJC04CjbHixz
2xV0KyC4NNo64g+2nlDFJKYQPwQSKdDFaci9cGwY=
Date: Wed, 30 Mar 2016 01:38:03 +0300
To: ...@gmail.com
Subject: =?utf-8?B?0J/RgNC+0LLQtdGA0LrQsCDQvdCwINC00L7RgNC+0LPQsNGF?=
User-Agent: Heirloom mailx 12.5 6/20/10
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Message-Id: <20160329223803.13534100BE@mail.aqua-mail.com>
From: ...@aqua-mail.com (root)
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