Well, if you've added oak to the primary I'd taste it when going to the secondary so that you'll know or have an idea as to when to rack off the oak.
Otherwise, you can taste anywhere along in the process. IMHO you won't know the quality of the batch until it's had time to mature like in a year or so.
I take hydrometer readings and taste during the later part of secondary fermetation. I've never sweetened a wine yet. The gooseberry wine I'm making (0G 1.102) will have a high %alcohol if fermented to dryness, so I'm going to try and stop it early for a semi-sweet wine. When tasting, the sweetness is countered by tartness and astringency that might mellow after aging. Is there a SG range that would be a good target for a semi-sweet/semi-dry fruit wine?
So I only taste at the first real racking (so not from primary to secondary).
And then I taste when I am going to bottle.
I always presume that if I made the calculations right, and there are no visible deseases in the wine it will turn out fine.
The only time I lost 2 batches was last year when I went on a holliday and the
summer turned out really hot. Above 32 degrees celsius for several days on a row.
When I returnmed from holliday I ended up with 2 10-liter batches that turned into
syrup with a bit of alcohol. I dumped them.