It is, the industry standard is to tip on pick up $1-2. I'd go with $2 and adjust upwards for any special situations, service, rain, etc. Some also tip on drop off, this is a personal decision and not expected.
We use the valet often. I always tip both ways usually $2 eachway, more if its super hot or pouring rain. Staying dry and having a cool car are worth a few $$ to me.
We use valet, especially the first day when we have groceries and luggage and the last when we have...well, everything! We tip both ways, $2-3 usually, unless there are a lot of suitcases or bags, in which case it's $5.