$\sin{x}$ or $\mathrm{sin}\ x$

Compare:

\documentclass{article}
\begin{document}
\[ x\sin^{2} (t) + x\cos^{2} (t) = x\]
\[ x\mathrm{sin}^{2}\ (t) + x\mathrm{cos}^{2}\ (t) = x\]
\end{document}

in my opinion, the first option is much clearer.

Considering also my registration on Math.SE., any user can adopt the long notation \operatorname{...} (eq. n. (3)) which provides the same white spaces as the formula n. (2). \operatorname{...} can be used to define any mathematical operator, with the correct spaces, when yo're using MathJaX.

You can observe that the (2) (it has been used \sin and \cos operators) and for the (3) \operatorname{...} and they are the same. It is visible that using \mathrm{sin} with severals \, you have note the correct blank spaces.

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}
\usepackage{xcolor}


\begin{document}
\noindent Limited spaces for the operators:
\[\color{red}{\mathrm{sin}(\alpha+\beta)=\mathrm{sin}\alpha\mathrm{cos}\beta+\mathrm{cos}\alpha\mathrm{sin}\beta} \tag{1}\]
Correct spaces for the operators:
\[\color{green}{\sin(\alpha+\beta)=\sin\alpha\cos\beta+\cos\alpha\sin\beta} \tag{2}\]
Correct spaces for the operators: same of the previous code:
\[\color{green}{\operatorname{sin}(\alpha+\beta)=\operatorname{sin}\alpha\operatorname{cos}\beta+\operatorname{cos}\alpha\operatorname{sin}\beta} \tag{3}\]
Additional spaces given by the control \verb| \ | showing a bad spacing:
\[\color{red}{\mathrm{sin}(\alpha+\beta)=\mathrm{sin} \ \alpha \ \mathrm{cos} \ \alpha+ \mathrm{sin}\ \alpha \ \mathrm{cos} \ \alpha}\tag{4}\]
\end{document}