I do usually wash everything because it just makes me feel ready, but I also buy our clothes and don't really receive gifts because I never had a shower. I guess if you receive gifts that maybe you didn't pick out, etc then it might be best to keep the tags on things.
I would wait with the newborn stuff until you are about a month or so about from giving birth to see what baby weights. Newborn is 5-8 lbs. usually so it may truly be wasteful to take tags off.
I was working on washing everything yesterday.. it was a lot of work... I learned a valuable lesson though.... If you are buying someone a gift, read the washing directions.. If it reads, wash separately.... DO NOT BUY IT. Most things were fine but I had a couple of gifts that are very nice but have to be washed separately so I found that frustrated.....
I bought two large garbage bags of used clothing... We have well over 200 something items for the 0-3 month range... Good thing some of them are pretty ugly and I wont feel bad if they are not worn... I just had to do it because they were so much cheaper, most are in like new condition and I was too frustrated with the trying to figure out if what i have is enough and how much more I need.. Now I know I definitely will not be short on anything.
Oh and the other part of your question not everything gets worn if your lucky like I was we had a million outfits. I ended up about once a month waiting until my daughter was in a good mood and changing her 10 times so I could get pictures of the cute outfits grandparents had sent. Even though she never really got around to wearing all those dresses more than once.
Washed and worn once you can sell them to a second hand kids store and pick up the sizes you need on trade. There are some high end stores that sell almost brand new baby clothes.
Yes wash them all in a baby detergent like Dreft or All Baby. Everything including blankets and burpers. It's a good idea to also wash your nursing tops and bras in that same detergent to keep from irritating baby's skin.