If you publish, then you should referee as well. The issue is how much. There is no written rule, but here you can find a good receipt. I did follow this receipt till 2021, when I decided to stop given the enormous increase of review requests and, in particular, those asking a report within unacceptable deadlines. As for the past:
2021: 09/21/43%
2020: 12/30/40%
2019: 15/28/54%
2018: 14/23/61%
2017: 15/19/79%
2016: 20/34/59%
2015: 14/18/78%
2014: 15/20/75%
2013: 25/35/71%
2012: 22/25/88%
2011: 14/17/82%
2010: 10/15/67%
2009: 15/16/94%
2008: 05/07/71%
2007: 05/07/80%
2006: 03/05/60%
2005: 05/07/71%
Since 2005, I have reviewed 1 book proposal and 209 papers out of 317 so 66%. In 2011-2014, I also served as a reviewer for Mathematical Reviews and for several foreign (EU and non-EU) project proposals. Below a list of journals and conferences which I refereed for.

Abstract and Applied Analysis
Acta Biotheoretica
Acta Scientiarum Mathematicarum
Annales Polonici Mathematici
Applied Mathematics and Computation
Applied Mathematics Letters
Applied Mathematical Modelling
Applied Numerical Mathematics
ASME Journal of Dynamic Systems, Measurement and Control
Bullettin des Sciences Mathématiques
Communications in Nonlinear Science and Numerical Simulation
Comptes Rendus Mathématique
Computer and Mathematics with Applications
Electronic Journal of Qualitative Theory of Differential Equations
Engineering Computations
European Journal of Control
IEEE – Transactions on Automatic Control
IET Control Theory & Applications
IMA Journal of Applied Mathematics
IMA Journal of Mathematical Control and Information
International Journal for Numerical Methods in Engineering
International Journal of Bifurcation and Chaos
International Journal of Control
Journal of Biological Systems
Journal of Computational and Applied Mathematics
Journal of Computational Dynamics
Journal of Difference Equations and Applications
Journal of Differential Equations
Journal of Dynamical and Control Systems
Journal of Mathematical Analysis and Applications
Journal of Mathematical Biology
Journal of Scientific Computing
Journal of Vibration and Control
Journal of Zhejiang University Science
Mathematical Biosciences
Mathematical Biosciences and Engineering
Mathematics and Computers in Simulation
Mathematical Modelling and Analysis
Mediterranean Journal of Mathematics
Nonlinear Dynamics
Numerical Algorithms
Numerical Methods for Partial Differential Equations
Physica D
Rendiconti del Circolo Matematico di Palermo
SIAM Journal on Applied Mathematics
SIAM Journal on Control and Optimization
SIAM Journal on Mathematical Analysis
SIAM Journal on Matrix Analysis and Applications
SIAM Journal on Numerical Analysis
SIAM Journal on Scientific Computing
Systems & Control Letters
Transactions of the Canadian Society for Mechanical Engineering

American Control Conference (ACC)
IEEE Conference on Decision and Control (CDC) and European Control Conference (ECC)
ASME Dynamic Systems and Control (DSC) Conference
ASME International Design Engineering Technical Conferences (IDETC) and Computers and Information in Engineering Conference (CIE)
International Federation of Automatic Control (IFAC) World Congress (WC)
International Federation of Automatic Control (IFAC) Workshop on Time Delay Systems (TDS)