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 $246.75   13.156%   $279.2124   $279.21   $279.20   $279.20  
  Purchase #2 $101.29   12.203%   $113.6504   $113.65   $113.65   $113.70  
  Purchase #3 $114.32   3.516%   $118.3395   $118.34   $118.35   $118.30  
  Purchase #4 $12.99   4.176%   $13.5325   $13.53   $13.55   $13.50  
  Purchase #5 $171.34   5.278%   $180.3833   $180.38   $180.40   $180.40  
  Purchase #6 $171.55   7.717%   $184.7885   $184.79   $184.80   $184.80  
  Purchase #7 $101.47   4.111%   $105.6414   $105.64   $105.65   $105.60  
  7 purchase totals = $919.71       $995.5480   $995.54   $995.60   $995.50