Yes and no! You can train abs everyday as long as the volume is low (1 exercise 3-5 sets) and you are able to recover day by day. If you are training with higher volume (full ab workouts) you will need more time to recover between workouts. If you don't allow adequate time for recovery not only can it lead to injury but it can also lead to weakness and instability in lifts that require core activation and strength like squats, deadlifts and similar movements.

Training abs and having a strong core wont necessarily lead to having a visible six pack. In order to see your abs you must lower your overall body fat through diet and cardio/training. Unfortunately you can't just target your midsection when it comes to losing fat, you must lower your body fat across your entire body. See nutrition questions section for more information and helpful tips on dieting.

Your core is a group of more than 20 muscles in the lower back, stomach and hips. Your core muscles play a huge role in your everyday activities, from getting out of bed, to walking down the street, and bending over to pick something up, most importantly, they literally connect your lower body and upper body together and help you stay upright. Strengthening your core can lead to stronger lifts, improved posture and help prevent injury! The abs (rectus abdominis ) is a small fraction of this large and dynamic muscle group. ... Your abs sole function is to assist in bending the torso forward (Flexion of the lumbar spine).

Bottom line is that training any muscle with resistance will lead to it strengthening and growing (getting bigger & thicker). So if you feel that you have a wide waist to begin with I would avoid using heavy weights to train your abs and core (especially your obliques!). Throwing in some weighted ab exercises in between bodyweight exercises is fine, but if you feel your waist is a bit too thick for your liking, then avoid using heavy weights and focus on strengthening your core using bodyweight and pay close attention to your diet!

The short answer is YES! Training your abs during your offseason will just only make them more detailed once you slim down and enter your "cutting" phase. Its also very important and helpful to maintain a strong core throughout the year to help strengthen your lifts, prevent injury and improve posture.