You don’t choose which creature card you’re exiling, if any, until the triggered ability is resolving. When you do, the reflexive triggered ability triggers. Players may respond to this reflexive triggered ability, knowing the creature types of the card you exiled and which creatures might get a +1/+1 counter. This is different than effects that say “If you do” in that players may take actions after you’ve exiled another creature card but before putting counters on other creatures you control.