letters of rec

Definitely bring at least 3 letters to each house. The top houses won't even let you in if you don't have one.

you don't need any

Yes ad no. A general letter of rec may not do much other than increase the likelihood of your being matched with someone with a similar background. An rec from a current or alumni who is a direct relative however will qualify you in many cases as a 'legacy'. It doesn't increase your chances per say, but it may change the process.

Recruitment practices vary from house-to-house but in my experience we adhered to the following code of conduct:
-all legacies were always invited back after the 1st round
-if legacies are dropped 2nd round then it is best practice to inform the current or alumni family member either by email or phone
-legacies in the 3rd round are less likely to be dropped at this stage BUT if they are the recruitment director or house president is to immediately inform the recommending current/alumni member by phone
-any legacy invited to the final pref night was automatically given top bidding priority

Again all this could vary from house to house. In my house's case legacies were given an automatic pass first round in respect of the current or former alumni providing the rec. But there was a lot more pressure particularly in the 2nd round for the house make a yes/no decision in effort to avoid keeping a legacy PNM around until nearly the end and then just dropping them. Dropping direct legacies on pref night did not happen unless they chose to go somewhere else.