Pre-6 I use phos for bush-checks in matchups I can afford to get close enough to bushes to risk it, and to vision enemies that like to try and harass from bushes.
Post 6 I use it for the same pre-6 purposes but also as part of my kill setup not for any kind of damage, but to assure vision for rockets as they flee. Since I get an extra rocket's worth of mana out of having phos-bomb at level 1 this seems reasonable to me.
I level valkyrie first beceause valkyrie's mana cost doesn't increase, I want to be able to engage again after using valkyrie as quickly as possible assuming I'm still in servicable condition. Overall, I find that jump is better for producing a good burst of damage anyhow since it lets me more reliably hit with gatling and auto attacks.
I never use phos-bomb to push minions. Gatling is way more effective if I have to push using abilities, and worth the 20 extra mana.