I think it's totally about your comfort level (both emotional and physical-- 6 weeks post surgery really isn't very long and you may still be stiff or tender... A side zipper may not be the answer.)
A general note-- classic styles are classic because they work for everyone all the time. It will continue to work for you. The top that was your favorite top last year might not be your favorite top post-surgery, but you can definitely still do a classic style with an artsy edge.
Specifically, button-down shirts might not hang evenly without a reconstruction. Graphic prints (like stripes) will highlight the new contours. More fluid patterns will hide it. Depending on the cut and color, a solid color might highlight. A cardigan will break up those lines.
If your intent is ultimately reconstruction and you're just not sure if it's going to be at the same time as the mastectomy or if you'll need to wait a while, then there is absolutely no reason why anything about how you dress would need to change unless you just want it to. You can use a prosthetic bra to bridge you to reconstruction. A reconstructed breast might be noticeable in a swimsuit, or in a tank top or a very snug-fitting, low-cut top, but if you're talking about a sheath dress or a tunic it's likely that no one will ever know if you don't tell them and if they're staring at your chest they can see what they see. At least that's how I look at it.
If you're not planning on reconstructing, I think it introduces a couple more questions. Like, what do you want? Do you want to celebrate surviving and tattoo roses across your chest? (which, incidentally, I think is totally awesome, but I hope no woman ever feels like that's how she has to respond to that situation). Or do you want to put the experience behind you as quickly as possible and try to avoid drawing attention. How you feel about the change is important, and you will feel how you feel and how you feel is a valid reaction. You shouldn't feel like you HAVE to hide, but you also shouldn't feel guilty if you do.
Ultimately, what story do you want the photos to tell? If you don't want your surgery to be part of that story, add a cardigan or a shawl to cover up. If you want to talk about this as the year that you beat cancer AND saw your son graduate, wear a brightly colored snug top with a pink ribbon and just dare anyone to say anything (fyi- they won't, and they won't think anything except that you have courage they hope they never have to find in themselves).
And I completely second Laurel's wishes for a speedy recovery.
