Now I was wondering if the best option would be to continue the training, and add, say, +20 extra ev's to make sure I didnt get too few.
Yeah - that's probably what I'd do.
You could reset the EVs to 0 and start over, but I probably wouldn't bother unless they're pretty low as is. And it depends on the game - in the fourth gen, EV reducing berries are fairly easy to come by, but in the fifth gen, they're only available in the Dream World. And if you're in the fourth gen and have over 100 EVs, the first berry will take the total down to exactly 100, which would be handy if you've got something over 100 already. That doesn't work in the fifth gen though - it's just 10 EVs per berry until you get to 0. In the fifth gen, you can use some of the Join Avenue shops - most notably the Salon - to lower EVs, but it's not until fairly late in the game that you get access to those services.
So - yeah. I'd probably just keep going until I knew I had 255. You end up "wasting" 3 EVs that way (6 actually, since you'll end up with another 3 left over from the remaining ones), but that's not really that big of a deal, or at least it isn't to me - I know some people who get really bent out of shape over little details like that. If you're one of those people, then I'd think that resetting them is the only alternative.
*edit* - actually, I like molivious' idea (that went up while I was writing). If you train for whatever other EVs you were planning on first and make sure those totals are right, then you can come back to whatever you were training when you dropped your DS and just go until you're sure you've maxed it out, and it'll be 252.