Is it safe to downgrade Lion from 10.7.1 to 10.7?
Downgrade 10.7.1 (Client) to 10.7

Downgrade is possible, apparently without restriction, but not necessarily OK. 

My own case

After using 10.7.1 for a few days, a series of events led me to erase my startup volume then use Recovery OS Time Machine to restore from backup. These events left me without the Recovery HD that is required for FileVault 2. 

At that time, version 1.0.8 of the installer was not available. 

To regain a Recovery OS: 


avoiding my usual account, I used a different account to log in to 10.7.1 (intention: data integrity — avoid my usual home directory at all stages during a downgrade)
I ran version 1.0.4 of the installer on 10.7.1 to install (downgrade to) 10.7
avoiding my usual account, I used a different account to log in to 10.7
I observed an issue affecting a peripheral display — at least one thing not OK
I didn't investigate that issue, didn't seek other issues (intention: spend as little time as possible in a downgraded environment)
I ran Software Update, got 10.7.1 and other updates.


More generally

Data relating to the greatest version of an executable may be incompatible with, or lose integrity with, a lesser version. 

Incompatibility or loss of integrity may be acceptable for some types of file (examples: cache files, preference files). For other types (example: application support) integrity may be critical. 

For the System version/build over which you wish to reinstall, if Apple provides a matching full installer: 


avoid downgrade — prefer to install a matching system. 


Application-specific formats that are designed for export and import

To minimise the risks associated with downgrades, I guess that: 


an archive produced by Address Book in 10.7.1 should be usable without issue in 10.7 
an archive produced by iCal in 10.7.1 should be usable without issue in 10.7


– and so on.