Well, not actually posters, but A4 sized papers which will be changed regularly? Dd wants to do this for her AS revision. Her bedroom walls are newly painted and I would like to keep them in good condition if at all possible.
Could you put up a large pin board (or several) instead, hung on a picture hook? Blu-tack leaves greasy looking marks over time and the white version isn't much better. If the papers are going to be changed regularly, best to minimise contact with the actual wall coverings/finish.