You have to assume that each player wants a card each turn
Then you have to assume that each player will get a card via the cheapest available opportunity.
If you can protect all of your terts so as to not leave a single anywhere neighbouring a hostile tert AND someone else has left a single open in this way, you'll probably not get targetted, at least at first, for your singles being taken out for card spots.
However if someone else notices you're doing this, it may be that others will leave the board in such a way that the 3 stack you left to not get hit is actually the cheapest option for another opponent to take a card spot, they may leave a 5 or a 6 on their border and force someone else to hit you (it's something I do from time to time if someone is playing that way)
It's all very situational and there isn't a 100% cut and dry answer, but generally it's safer to play the way people expect you to play when you aren't trying to make ripples. By that I mean if you're biding your time, building your troops, and you're not ready to launch a big attack, then people will remember the player in the game playing "cheap tricks" like this, and you may be targetted where two options for attack are equal or nearly equal. You almost want to remain invisible for as long as possible (assuming you're playing standard or terminator games) and not provoke any sort of "grrr that git is playing THAT way huh" type reaction. In most games people will pull the odd stupid move and little rivalries will develop, if you play the way people are expecting you to play then you don't get involved in them so much and you will win more games because of this in the long run.