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 $244.94   13.331%   $277.593   $277.59   $277.60   $277.60  
  Purchase #2 $183.30   5.495%   $193.3723   $193.37   $193.35   $193.40  
  Purchase #3 $27.91   8.492%   $30.2801   $30.28   $30.30   $30.30  
  Purchase #4 $46.35   5.838%   $49.0559   $49.06   $49.05   $49.10  
  Purchase #5 $13.59   10.125%   $14.966   $14.97   $14.95   $15.00  
  Purchase #6 $41.59   4.809%   $43.5901   $43.59   $43.60   $43.60  
  Purchase #7 $37.76   10.384%   $41.681   $41.68   $41.70   $41.70  
  7 purchase totals = $595.44       $650.5384   $650.54   $650.55   $650.70