I use drinking straws and I have never had a problem, but I know a lot of people don’t trust them. Bubble tea straws work very well too. I try to make my cakes as level as possible before doweling. Once I insert a straw I cut it flush with the iced cake. Then I remove it and use it to cut the rest. If your supports are different lengths, your cake will always be “off”. If there is a little space between the straw and cake, just fill in with buttercream. Since I use straws I just use scissors to cut, but when I have used plastic columns in the past I used garden pruning shears (that I bought just for that purpose). Good luck!