It shouldn't have any bad effect on your account but deleting your old posts might affect your forum ranks. I heard someone with full member rank deleted his few old posts and it ranked down.
That will happen if the user who deletes his posts old posts deletes so many of those that less than 120 posts remain. 1 post = 1 activity point and if someone who has 120 activity points and 200 posts and suddenly that person decides to delete 120 posts then he/she will have only 100 activity points which will de-rank that member. A full-member rank requires one to have at least 120 activity points.