If programming languages is important for biologist then is better to learn in undergraduate or postgraduate study, or both? I think it is important for both.
Learning Programming language is necessary if you have such type of work which require programming. Studying a programming language will help you in future to interact with the computer in your area of research. Just like mostly now a days the doctors and hospitals are using SPSS to process their data. SAS is also used and some time they use NVIVO. A lot of software's are available depend on your requirement of research. Students of BIOINFORMATICS mostly using merge of computers and Biology in their fields. Programming languages which are used now a days are C,C Plus Plus, Java, Python etc etc
Why limit the study of computer programming to those involved in academia? Learn computer science by taking courses, but learn computer programming by self-study, experimentation and practice; with a book on a reading stand and a keyboard at your finger tips. To start I suggest limiting the choice of languages to C and/or Java, the latter for the Internet, but either can be used for a wide variety of programming purposes, depending only on your own imagination. Just buy and use an appropriate book. Download the programming software, without cost, and install it on your computer. Also download any zip files that contain the examples for your book.