As an independent contractor, does my hourly rate determine if I should charge GST/HST to the client? Or should I charge it always?
Hourly rate is not the determinant. You could be selling widgets, not hours.
Rather, there's a $30,000 annual revenue threshold for GST/HST.
If your business's annual revenues fall below that amount, you don't need to register for GST/HST and in such case you don't charge your clients the tax.
You could still choose to register for GST/HST if your revenues are below the threshold, in which case you must charge your clients the tax. Some businesses voluntarily enroll for GST/HST, even when below the threshold, so they can claim input tax credits.
If your annual revenues exceed $30,000, you must register for GST/HST and you must charge your clients the tax. FWIW, certain kinds of supplies are exempt, but the kind of services you'd be offering as an independent contractor in Canada aren't likely to be.
There's more to the GST/HST than this, so be sure to talk to a tax accountant.