Ok,
When I say "always" do a take-off performance calculation before accepting a load, obviously the one you've done at the beginning of the season to establish that your home runway has more length than you could possibly need can be something 'known' rather than something which needs to be recalculated each time.
I think all ag-pilots should at least be 'familiar' with the calculation, even though they might use other techniques. The trouble starts when a few of the variables start to change. i.e A different type of aircraft. A different strip. A product with an unusually high SG.
One of these on there own may not be enough to spoil your day but you only need one occasion where 'all the holes line up' to completely stuff your career.