For tech startups, are trademarks necessary?

You've received some great answers so far, especially from Mr. Mincov. When you say that you are a "tech startup," I take that to mean that 1) your resources for investing in the establishment and enforcement of marks is limited and 2) marks and branding are not as important for selling your sophisticated and technical goods/services as they might be for goods which appeal to consumers based more upon subjective and emotional criteria. What is important for you to know is that you do accrue rights in a mark in the U.S. merely by using the mark; and registering that mark is not necessary in order to enforce your rights in it. However, a U.S. registration can reduce the cost of enforcing your rights since it 1) provides notice of your rights and is an effective deterrent against others using your mark; 2) becomes a form of evidence of your ownership and the validity of the mark; and 3) extends your rights in the mark throughout the U.S. instead of in only the markets where you actually use the mark. However, in many countries outside the U.S., you do need to register a mark in order to have any rights in the mark and in some countries, a mark needs to be registered in order to be able to use it.
That said, even without registering your marks I believe it is important and highly recommended that you at least initially consult with IP counsel to do your due diligence and ensure that you are using marks which are available and not at risk of infringing any other marks; you are using a mark that is protectible as a mark; and you understand what trademark or service mark "usage" is because trademark "use" is not a concept that is as straightforward as it seems. As Ms. Nichols alluded to, owning a domain name does not establish any trademark rights and is not itself use of a mark in Commerce. Only owning a domain name, posting marketing content on the referred website (if a service) and advertising the domain name as a mark is service mark use which establishes service mark rights. For goods, you have to actually affix the mark to the goods or its packaging in order to use a mark sufficient to accrue rights in it.
Moreover, you need to understand what is a distinctive mark which can acquire trademark/service mark rights v. a non-distinctive or generic mark which cannot be protected as a mark. Also, you mention using multiple iterations or variations of a mark or marks but will there be some consistency or element of those iterations that are sufficiently constant that consumers will recognize the marks as emanating from the same business and knowing that business is yours? Because if your mark changes too frequently such that consumers lose the ability to recognize the mark as identifying your particular company, then you effectively don't have a mark. The purpose of a mark is to identify your business as the source of the goods/services.
You want to make a sound business decision about this but you need to consider both long and short-term costs associated with your decision and investing in some expertise on this issue might be worth your while to avoid additional costs down the road.
*Please note my disclaimer in my bio.