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 $12.70   3.282%   $13.1168   $13.12   $13.10   $13.10  
  Purchase #2 $76.40   6.656%   $81.4852   $81.49   $81.50   $81.50  
  Purchase #3 $51.59   11.733%   $57.6431   $57.64   $57.65   $57.60  
  Purchase #4 $135.62   3.249%   $140.0263   $140.03   $140.05   $140.00  
  Purchase #5 $97.99   13.671%   $111.3862   $111.39   $111.40   $111.40  
  Purchase #6 $26.81   7.494%   $28.8191   $28.82   $28.80   $28.80  
  Purchase #7 $186.30   10.612%   $206.0702   $206.07   $206.05   $206.10  
  7 purchase totals = $587.41       $638.5469   $638.56   $638.55   $638.50