For those adding seat selection onto a Tango fare - check the difference between Tango and Tango Plus. I booked a flight from YYZ to YVR a few days ago. Seat selection would have been $22 and to "upgrade" to T+ was about $35. So, I got my seat selection, plus 100% status miles, and cheaper change fees for about $14 more than just seat selection.
Also, consider only selecting some seats. If, for example, you have two adults and one child traveling, and you want your child to sit next to one of you. Consider selecting two seats (child plus 1 adult). The second adult may end up sitting away from the pair, but your child is with one adult and you save one seat selection fee. CAVEAT - I've not done this with AC, so I don't know if you can take different options for different passengers, but it works with other airlines. You could do two reservations and then link them, if you cannot do it in one reservation.
Also, consider only selecting some seats. If, for example, you have two adults and one child traveling, and you want your child to sit next to one of you. Consider selecting two seats (child plus 1 adult). The second adult may end up sitting away from the pair, but your child is with one adult and you save one seat selection fee. CAVEAT - I've not done this with AC, so I don't know if you can take different options for different passengers, but it works with other airlines. You could do two reservations and then link them, if you cannot do it in one reservation.