In practical terms Vref 40 is usually only about 7 KTS less than Vref 30 and given that Vref for current flap setting is also V2 for the go-around there is sufficient margin to continue with Vref 40 speed but with flap 30 selected under normal circumstances, usually we set 5 its on top in the MCP, but also consider if we do a dual channel approach the Vref is commanded as per the FMS speed, so in a nutshell there may be only 2 Kts difference. Obviously if we decide to land Flap 30 then the designated Vref should be set. As commented, selecting F25 can drag the speed down. The other consideration is if you are near the speed limits and the wind shifts you could end up auto-retracting from 40 to 30 in any event.