Ascorbic acid can definitely affect the taste of your wine.
Although it is a good antioxidant, it is also a rather harsh acid on the pallet. You need to be rather careful and always do a bench test (add a bit to a small sample, taste, and scale up from there.
The only time I ever use ascorbic acid is when I am dealing with latent sulfur issues. Ascorbic acid can convert mercaptan compounds into H2S which can be easily eliminated through the use of applied copper.
Other than that very specific use, I never add ascorbic acid to my wines. It is just too harsh.
There is a much better approach to controlling oxidation. If you maintain the right level of SO2 given your preferred PH level, and keep the carboys sealed with a minimum of headspace, you will definitely keep oxidation in check.
I am surprised that your shop owner made this recommendation, especially when we are dealing with a red wine which is already loaded with antioxidants.