Hey Tannu,
On our end (Stripe) the line items are sorted by reverse chronological order (most recent first) and end with the line item for the subscription if any. This was explicitly released behind a new API version: 2015-04-07 [1]. Before that, the order of lines was inconsistent which could be confusing.
There's currently no way to reverse the order on the invoice or the associated receipt. The best solution here would be to send your own email receipts so that you can display the lines in the order you want.
Another solution would be to use a $0 plan for everyone. You would then create an invoice item [2] each month first for the tax (or let Avatax do it) and then for the subscription's price. This way you'd force us to have the first row be the one that corresponds to the plan amount itself and the second row be the tax. The caveat is that, with a $0 plan, there'd still be a third line item for the $0 subscription that the customer is on at that point so you'd really see three lines on the receipt: line item for the plan price, line item for the tax, line item for the "fake" $0 plan.
Hope this helps!
Remi