I have also used hidden pillars for years (I stopped using dowels long ago), and I have never had any issues with weight on them. I have done up to a 5-tier fondant cake (with filling in each tier!!!!!) with them, and there were no problems at all. I think they are so much better (and so much easier!!!) than dowels!! I think you can rest assured that the cake will be perfectly stable! 
BETHROSE, Push-in pillars and Hidden Pillars are not the same thing. the push-in pillars are tapered, and while they are quite strong, your cake and filling must be sturdy. I managed 3 tiers, but the fourth one started to wobble, and would have fallen. I put the last two tiers on the table.
Hidden pillars are excellent. They allow a small gap between the tiers to allow flower fillers. they are very strong and very stable, stack away.
I used the crystal look push in pillars for a three tiered wedding cake last year. It seemed to work fine, but I am going to be setting this cake up early and leaving to do another wedding, and didn't want to have to worry about the leaning tower of wedding cake. The pillars would be in a 12" red velvet with creamcheese filling, not the most sturdy of all the cakes. Should I plan another construction idea?
This cake is to feed 100 people including the top tier. She wants half redvelvet and half vanilla. I was putting the redvelvet on the bottom because it is a heavy cake that seems to sink in the middle. The vanilla bakes more evenly and I can stack them better.
Any ideas would be appreciated.
Bethroze, I highly recommend the hidden pillars (made by Wilton, 4 in a package, here's the link so you can see them: http://www.wilton.com/store/site/product.cfm?sku=303-8&killnav=1 ).
Like I said, they hold A LOT of weight. I don't know of any stacking system (dowels included) that is more stable. Also, if you're not putting flowers in between the tiers, you can trim the pillars down so that the separator plate sits flush with the cake, and then they truly are hidden.
I was wondering the same thing about the push in pillars. I have never used them but I have done what cakesbyjess suggested. If you look in my photos the snowflake cake is done that way. I guess it all depends on what kind of look you want for your cake.
Lousaria ... your snowflake cake is stunning!!! You did a wonderful job! What are the silver plates/boards between each tier??
Thanks Lousaria ... they really do look perfect with the winter-themed cake!!
sdfisher ... that 6-tier cake is gorgeous!!!! Wow!!!
I have used the push-in pillars for many cakes. For added reassurance, I also use a plastic dowel rod cut to the same height as the push-in pillars, and place it at the center of the cake as one more "leg." It helps distribute the weight, and helps calm the wobbels - the cake's and my nerves'!
Thanks everyone...Everyone at this site is such a big help. What did I do before a friend told me about it? Oh yeah, house work! 
I was wanting to use what I already owned, but looking at the cake I used my pushin pillars with, I might want to go with shorter ones. I had to make that cake tall to not look dwarfed by the buffet mirror behind it. I will still be changing my ming until the day I put it together. 
Isn't this site great?!?!?!
Glad we could help ... good luck with your cake. We'd love to see pictures of it!
hidden rods are so great and the push ins are a little wobblie iused the hidden and iced them didnt try to hide them at all looked great
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