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 $29.58   3.799%   $30.7037   $30.70   $30.70   $30.70  
  Purchase #2 $101.74   7.222%   $109.0877   $109.09   $109.10   $109.10  
  Purchase #3 $28.25   6.162%   $29.9908   $29.99   $30.00   $30.00  
  Purchase #4 $30.59   12.737%   $34.4862   $34.49   $34.50   $34.50  
  Purchase #5 $133.55   4.117%   $139.0483   $139.05   $139.05   $139.00  
  Purchase #6 $148.97   7.492%   $160.1308   $160.13   $160.15   $160.10  
  Purchase #7 $60.64   11.815%   $67.8046   $67.80   $67.80   $67.80  
  7 purchase totals = $533.32       $571.2521   $571.25   $571.30   $571.20