Designing a new logo, or even updating an existing one can become a complicated task very quickly. The identity of the company rides on the exact perception of the new mark. Many rounds of revisions can focus on icons, color, fonts, spacing, placement and all the way down to the kerning of the letters. Every detail is taken into account to summarize the company and its persona in a single mark. There are times when that mark has been so painstakingly focused on that the simplest thing may be ignored until the last moment. The trademark.
While it is important to design the trademark into the logo design (if it will be used) there are many times that it is added in the final stages and hasn't truly been incorporated into the overall design. When this happens, it can sometimes feel as though it was an afterthought especially when utilized in many differing cases. I had a contact reach out recently who seemed to be in this very predicament and I offered some simple advice.
It is not required to use the trademark symbol in every instance in order to protect your logo or tagline. If it is a registered trademark, it is protected no matter what. If it was designed with the TM in mind and is non-obtrusive to the overall aesthetic then you should feel comfortable using it wherever you feel it is appropriate. Having the symbol can exude a sense of exclusivity and strength, and in some businesses that can be very beneficial.
Size comes into play most frequently. You should not assume that the TM will be the exact size relation to the logo at all sizes. It may be a different ratio on a business card to that of a large brochure cover or signage. You don't need it to be very large for people to recognize what it symbolizes, but you don't want it to be so small that it appears to be a blob either. Using it on a web site can be overkill as well. Anything where you have an opportunity to put legal copy that discloses your copyrights and trademarks are good places to leave it off. Again, that's only if you feel that it is an 'add-on' to the overall design and doesn't simply blend well with the presentation of your mark.
In text, and this is most important, my overall rule for the use of the symbol is to not overuse it, such as multiple times in text like say a web site, trifold, sell sheet or in emails, etc. Use it once at the beginning to establish the point, and then leave it off subsequent cases. Where it may come into play being used multiple times can be in separate sections of a lengthy brochure or on separate pages within a site.
What it comes down to in the end is the strategy you decide on as a company. The better you define the rules around the logo's use are as important as the logo and tagline itself. Plan for how you want the designation to be understood and perceived and create a section within your brand guidelines to give clear direction to everyone within your company to know when and where it is appropriate, and when it is not.