Few staff in wholesalers are expert colourists so they wouldn’t be my first choice for colour advice, especially as I’ve overheard some very dodgy advice in the past.
What percentage of grey are you working with?
Obviously, if it was classed as 100% grey it would actually be more or less white.
Second important consideration, what’s the natural level?
If you’re trying for white, depending on the natural level and percentage of grey, I’d generally go for bleach then tone afterwards, but it depends on your starting point. You will usually find that the percentage of grey isn’t evenly spread so you might have lighter temples, top and the back area is much darker. Therefore, you might need to prepare a different colour mix to deal with different sections.
Finally, think about the depth of colour pigment in the colour you want to use as a toner. Remember that a level 10 contains less colour pigment than a 9. A level 12 highlift tint has even less colour pigment in it, so it’s unsuitable for using as a toner and really, not much use on anything darker than a natural 8 unless you want a warmer finish.