I recommend at least having a look at the derivative regardless of what the transformation or function is. It points to a lot of features that you should be signposting on your graphs, as markers will be looking for them (and they will often be included in the question).
There's not a lot of tips I can give since personally it's mostly based on intuition. But at least try to:
- Know key features of common transformations and how to identify them. I'd include exponentiation, reciprocation, simple translations, dilation, logarithms, powers.
- Practice
- If in doubt, as a last resort, plot some points
It doesn't matter what I say or think about time allocation for particular questions because different students excel and struggle with different topics. In general, you should be allocating more time for objectively harder questions, then subjectively harder questions, then whatever else is left. If it works for you go ahead, but you should try to experiment and see what exam strategy lets you maximise your marks; if it so happens to be leaving sketching questions last use it, otherwise try something else. This goes for every other subject too. As an example, I didn't spend a lot of on sketching because I left my time for the questions towards the back end of the paper. Use your reading time well too; you should be identifying what questions you want to spend more time on (sketching, if need be in your case) and planning accordingly.