why is it bad to suicide pref?

If you suicide (i.e. single intentional prefer) you are not eligible to be a quota addition. This means you will not get a bid unless you are high enough on your SIP's pref list, and there's really no way to know where you are ranked. If you list both houses, and maximize your options, you will receive a bid. QAs are RFM's (release figure methodology) way of rewarding those who maximize, so to speak. If you list ABC first and DEF second, and even if both ABC and DEF hit quota of 50 (for example) and you are not high enough on either list (you're #51 and 52 on the lists respectively), you are guaranteed to be placed. Where depends on how things are handled by the recruitment team. Sometimes it would be to your first choice ABC, sometimes it would be to the smaller group.

Plus you need to maximize choices each round, not just at pref. Your Rho Chi will help guide you.

Bump