Definitely use the boosters you get for free when grinding, but if you're thinking about buying them for gold you're almost certainly better off just using the gold to convert XP. I think there were various threads when boosters were introduced that came to that conclusion.
Comparing premium time vs converting XP... Take your average games per day, multiply by your average XP per game, and divide by 2. That's the extra XP you're getting each day for having premium time. 360 days of premium costs 16K gold, or about 44 gold per day. 44 gold converts 1.1K XP to free XP. So if you're getting more than 1.1K XP extra because of having premium time, then that will be a cheaper route for getting the required XP assuming that you don't mind the grind. Or if you don't want to buy premium time in such large amounts, you can similarly calculate how much extra XP you need to be getting from the smaller amounts of time to make it cost-effective.
If you're really desperate to get XP as cheaply as possible, you can probably buy premium time in tiny amounts when you're able to grind like crazy, use the various certificates you get from missions for 1 hour or 9 hours of premium time, use whatever boosters you get from missions and such to accelerate the XP accumulation, use free XP boosters on anniversary tanks, etc.