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 $48.84   13.847%   $55.6029   $55.60   $55.60   $55.60  
  Purchase #2 $49.35   5.466%   $52.0475   $52.05   $52.05   $52.00  
  Purchase #3 $140.90   7.651%   $151.6803   $151.68   $151.70   $151.70  
  Purchase #4 $73.81   4.224%   $76.9277   $76.93   $76.95   $76.90  
  Purchase #5 $235.77   5.543%   $248.8387   $248.84   $248.85   $248.80  
  Purchase #6 $169.89   8.813%   $184.8624   $184.86   $184.85   $184.90  
  Purchase #7 $202.32   4.489%   $211.4021   $211.40   $211.40   $211.40  
  7 purchase totals = $920.88       $981.3616   $981.36   $981.40   $981.30