Yes, you can whitelist or blacklist by subject line. Subject line-based whitelisting or blacklisting works better in some circumstances than in others.
For example, if you have a contact form at your website that always uses the same subject line ("Web Lead From xyz"), and some of those messages are being incorrectly classified as junk mail, then the subject line based whitelisting can work well. Similarly, subject line-based blacklisting can be effective with well-defined targets.
However, trying to come up with lists of phrases for subject lines that you think are spammy, can be time-consuming and counterproductive. For example, some companies may want to blacklist anything with the word 'sex' in the subject line. However, they might end up blocking messages that they actually want, while actual spammers might obfuscate the word by spelling it 's3x' or 'sexx'. In the end, it is often better to trust the work that has gone into developing the filters than to try to reinvent the wheel.
Therefore, our general recommendation is to use subject-based white listing and blacklisting judiciously.