Its better to filter it before injection to avoid very large clumps, but single cells can still clump within syringe...so just tap the syringe just before injections.

First, please note that tail vein injections of tumor cells are NOT considered as a spontaneous metastatic models. Rather, this type of an experiment is referred to as an Experimental Metastatic model. A spontaneous designation is used when an orthotopic model is in play. With MDA-MB-231 cells, an injection into the second thoracic mammary fat pad nearly always gives rise to spontaneous pulmonary mets, i.e., lung mets, as well as sentinel lymph node mets.

With respect to your question. I've not ever needed to strain trypsinized 231 cells. In our hands, 231 cells lift off the plates as a single cell suspension, during trpsinization. A problem may arise if you wait too long between trp treatment and syringe prep and injection time. During the delay settled cells may clump. Therefore, I suggest planning your experiments in a manner that allows for only a short delay between trpsinization and injection. During this time you may transport the cells on ice and keep them suspended by gentle flicking of the tube with your finger. Good luck.

I share at 100% Goodwin's and Paul's comments.

And, yes, indeed, injection of cancer cells into the tail vein of mice relates to "pseudometastatic models" (see the Mathieu V. et al. attached articles).

Orthotopic (see Mathieu A et al., Ingrassia et al.) or even subcutaneous (see Darro et al.) injcetions of certain cancerc cell types can lead to actual metastases.

Best regards