A He analyser may not be absolutely necessary. However, after topping up leftovers 5 or 6 times, you start wondering how much He there really is in there...
I ended up getting one. Those sold by
Tech Diving Limited in the US are good and compact and more often on sale than not, so reasonably priced. I just looked now and at the moment they are at $809. I got mine for much less than that though (I think $650 or so).
The older Atomox models may seem tempting (low price), but there have been lots of problems with orders not being delivered or being massively delayed. Beware. More info from disgruntled customers on the deco stop.