Device level scan mode?

Hello,

In Kepware ( KepServer Ex 6 server configuration ) what is the suggested Device level scan mode? What settings are appropriate to scan 1000s of tags under each device ?  

• Request all data at Scan Rate Vs  Respect Client Specified Scan Rate

For example , In our production system   I observed that "Respect Client Specified Scan Rate"  scan mode some opc tags are bad eventhough they are good on the source server side . After changing to " Request all data at Scan Rate",  most opc tags quality went from bad to good . 

What are the pros and cons of the above settings on overall impact