I've been sitting with both feet on the floor on a very low stool for about a half a year now and I think that there are a lot of benefits to sitting this way. I spent a good amount of time sitting with both feet on the rungs which I liked because it made me feel like my body was more symmetrical when playing, but I eventually found that I felt too perched up on the stool and that I didn't really have much of a feeling of contact with the bass and the floor. By sitting very low to the ground and with both feet on the floor I feel a more intimate connection with the instrument where I can feel the vibrations better which I think has helped me cultivate a better sound. Having some psychical connection to the ground is important to me and I think that it's important to have at least one foot on the ground or else it's very hard to get the feeling of being grounded while playing the bass. I can also sit further on the edge of the stool, more on my sit bones, which helps me focus my weight forward. I do not have the bass at a complete forward position in front of me however and have it more at an angle. I can use my left leg to adjust the angle of the bass in case I feel I need more access to a side of the bass. Since most of our playing is on the G and D strings, I don't see why we shouldn't favor them when we sit.
Another thing you might want to look into is angling your stool a little forward. I have a metal stool that is adjustable with screws so I lowered the two front legs down one notch so that the stool is sloping slightly forward. I feel that this makes it easier to have a straight back while playing and it makes playing in higher positions easier, which is difficult for me right now because I'm playing on an old French bass that has very large shoulders. I've only done this recently, but I think that I'm going to keep it this way. The important thing is to experiment and find what is right for you and your body. What's comfortable for one person might not work for another person so it's important to find what's personally best for you.