Letters of recommendation play an important role for admission into graduate school. Writing letters of recommendation is part of my job and I am happy to help students launch into the next step of their careers. While I am happy to do this for undergraduate research assistants, please consider the following:
In the summer before you plan to apply to graduate school (applications are typically due between December-January), talk to the graduate student you work most closely with in the lab. Tell them about your plans and hear what they think. Listen to their feedback and adjust your plans accordingly.
A letter of recommendation is not a blank check. In a letter of recommendation, I am recommending you for a specific program (Ph.D. M.S.W., M.F.T, etc) and providing evidence of why I think you would do well in that program or getting that degree. For instance, while I may be excited to recommend you for a M.S.W., I may have reservations for recommending you to a clinical psychology Ph.D. program. The qualifications needed and the careers desired between these types of degrees are very different.
I often get asked to write letters of recommendations for students who take my undergraduate courses (but who have not been an uRA in my lab). In regards to admission into graduate school, I personally advise against asking a professor of a class you took to write you a letter of recommendation. Why? Even if you were the best student in my class, raised your hand often, talked to me during office hours, etc., all I will be able to say is that you are a good student (which could have probably been captured by your GPA). In contrast, for excellent uRAs, I can say that I have worked with them for a year, I have observed that they are professional in working with families and young children, are reliable and meticulous, are engaged in the scientific research process, may be developing their own research questions, etc. In other words: most letters of recommendation are positive. What differentiates them is the degree to how positive they are. A positive, but somewhat vague and short letter of recommendation may hurt your overall application. If you have not worked with professors or other professionals related to the degree you want to obtain during your undergraduate time, you are most likely not ready to pursue that degree.
When you request a letter of recommendation, address the following:
What you plan to apply to and why, what your career goals are, what you have learned in the lab that makes you think you are ready for this particular graduate training, CV/resume, and a request if I am willing to write the letter.