If the DLM makes any changes to the line, then it should automatically force a resync so that any changes up or down are applied immediately.
As LL mentioned performing a resync at any time is entirely dependant on your current SNR margin. For those lines which vary over the course of the day in most cases the SNRm is lower in the evenings, so performing a resync at this time may give a lower sync speed.
A (rarish) case where it can be beneficial to perform a resync is if a line has been through a noisy patch where there has been lots of bit-swap. Bit swap can make the router mark certain tones as unusable. A line is supposed to have a certain amount of surplus SNR built in to cover bit-swap but if its been happening over lots of tones, then the overhead allowance gets eaten away at. If high bitswap carries on then at some point the router will perform a resync itself, but if the noise has cleared then performing a resync should bring those tones back into use.
TBH this doesnt happen nor is it needed often, as the router will have either done it by itself, but sometimes it may just get its knickers in a twist and start doing something silly. I suspect in some cases this may be linked in with the reason why occasionally if a line is seeing tons of errors, then a router reboot will clear it as the bit allocation table is started afresh during channel analysis which is performed as part of the sync up process.
Another time it can be beneficial is if you are with an ISP (such as PN) who apply a throughput rate restriction in line with either the sync speed or IP profile.
Because the BTw reports lag by a few hours, if you've had a positive change then you may have to establish a new PPP session to pick up the higher throughput speed. IIRC AAISP also restrict line rate, but based on sync speed rather than IPprofile so shouldn't impact quite as much.