Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $28.55   9.733%   $31.3288   $31.33   $31.35   $31.30  
  Purchase #2 $220.27   3.247%   $227.4222   $227.42   $227.40   $227.40  
  Purchase #3 $30.99   3.126%   $31.9587   $31.96   $31.95   $32.00  
  Purchase #4 $6.72   6.702%   $7.1704   $7.17   $7.15   $7.20  
  Purchase #5 $55.69   5.332%   $58.6594   $58.66   $58.65   $58.70  
  Purchase #6 $40.77   10.346%   $44.9881   $44.99   $45.00   $45.00  
  Purchase #7 $129.84   5.493%   $136.9721   $136.97   $136.95   $137.00  
  7 purchase totals = $512.83       $538.4997   $538.50   $538.45   $538.60