I never look at opponents previous games - but should I?

Hi all, I've been looking for chess.com rules on Daily/ 90min games, can't see anything on this topic.

I never look at my opponent's previous games to see their favourite openings and tactics, it feels like cheating (and as the old saying goes "you're only cheating yourself"). But players in big serious chess tournaments obvs study their opponents in exactly this way.

So what's either a) the rule, or if there isn't a firm rule then b) the etiquette on chess.com? Am I being too puritanical and missing a trick, or am I doing the right thing?

Obvs this only applies only longer time formats, more "classical".

'When you sit down to play a game you should think only about the position,
but not about the opponent' - Capablanca