is this useful to help break RSA?
Yes, this stems from elementary school arithmetic:
If you multiply two numbers ending in 1 the resulting least decimal digit or result will always be 1. Primality not required.
This does not however mean that if the resulting decimal digit is 1 both original primes end in one: e.g 7*3 or any primes ending in 7 and 3 respectively.
The best way to break RSA (excluding rubber hose cryptography) is to factorize the composite n. This is typically done using: https://en.wikipedia.org/wiki/General_number_field_sieve
Your observational on the behavior of the least significant digits though correct, does not obviously lead to any superior attack.
I use 2048 bit RSA and feel secure, and sometimes 4096 bits for extra safety margin. I expect this to remain this way for many years to come.