[Dovecot] Quota + Sieve

[Linux Advocate]

Thanx to everybody's help , i have got  a working combination of;

- dovecot LDA
- smtp auth thru dovecot sasl
- postfix
- virtual users in mysql
- postfixadmin
- thunderbird as the email client

Just 2 additional questions - 

A. Sieve
..............

With the imap filters in thunderbird, do i need to use sieve in dovecot? my initial observations/ findings say "no"?

B. Quota
.............

If i have not set quota in dovecot  or postfix ( no quota related configuration ), may i then assume, the users have "unlimited storage" , limited only by the hardware/filesystem?

I would like to have this feature ( just like unlimited storage in yahoo ) and just have a cron script running regularly to delete old emails ( for middle level, low level  users ). Is such a strategy ok?  

regards,
mgomez