The simple answer to this question is yes. If you have spent the winter months building your strength, power, and muscular endurance and you cease strength training when the race season starts you will slowly start to lose those hard earned gains and performance could suffer. Much will depend upon the type of racing you do. Sprint and standard distance athletes racing every few weeks might want to consider a maintenance programme with a couple of short sessions each week, rotating between strength, power, and muscular endurance in successive workouts. Even a 15 minute workout will be enough to preserve what you have. Athletes racing 70.3 or Ironman distance tend to race less, with longer gaps between events. This means it is possible to follow a strength routine most of the year, easing off 2-3 weeks before the key events and then resuming after a 2-3 week recovery period.
I would recommend maintaining a regular mobility programme throughout the year, as this will have a positive impact on movement efficiency and therefore technique, especially in the swim and run. In addition to this, a low level strength programme to protect ankles, hips and shoulders is also highly recommended.