there are theories either way, regassing will make very little difference to
the pressures IF it was still working before the regas.  All systems lose
gas over time, as the gas goes so does some oil, in theory water can be
attracted in via any leaks (I don't see how if there is sufficient pressure)
As the pressure gets low the system will start to freeze and the compressor
may cycle on and off more.  Very modern systems only contain a small amount
of gas, so any leak is more important.
On balance I would have it done every three years or so.