No harm in shaking every time, or at least if it has been sitting for weeks/months certainly years!
Well I've never shaken mine before, also wouldn't shaking it every time introduce more unnecessary oxygen that normally just sits at the top of the bottle? I always pour a full bottle of new Rodinal into one of those amber bottles when I open it (because the Adox Adonal lids on their plastic bottles suck) and then have a smaller bottle I work out of for convenience, so the larger bottle sits in the basement a few months at a time undisturbed. So that's why I asked, but when I pour off into the smaller bottle, I haven't noticed that my images have suddenly lost quality on account of pouring off the top without shaking... so that's why I asked.
~Stone | "...of course, that's just my opinion. I could be wrong."~Dennis Miller