Depends on how smart the spammer is, and how much they care. Most spammers a) are not very bright, and b) are in a volume business. Even the ones smart enough to do this may not think it worth the time to possibly improve a tiny percentage of harvested addresses. Especially since those of us who are dedicated enough to maintain tagged addresses are unlikely to respond positively to spam no matter what address we receive it on.

I use _, not "+" and anybody else might use "." or "-" depending on the configuration of their mail server. Sure, you can always add the full email and every possible stripped email to your lists, but in practice, few people seem to do that (judging by the amount of spam I get to me_randomstuff@example.net as compared to me@example.net).

I use "-" with an alias to "_". It turns out that when you give an email address to random people, they sometimes don't know the difference between "dash" or "hyphen" and an underscore.