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 $249.65   13.188%   $282.5738   $282.57   $282.55   $282.60  
  Purchase #2 $150.49   8.214%   $162.8512   $162.85   $162.85   $162.90  
  Purchase #3 $151.96   7.278%   $163.0196   $163.02   $163.00   $163.00  
  Purchase #4 $180.56   11.708%   $201.7   $201.70   $201.70   $201.70  
  Purchase #5 $45.71   4.307%   $47.6787   $47.68   $47.70   $47.70  
  Purchase #6 $124.43   3.827%   $129.1919   $129.19   $129.20   $129.20  
  Purchase #7 $176.54   11.203%   $196.3178   $196.32   $196.30   $196.30  
  7 purchase totals = $1,079.34       $1,183.3330   $1,183.33   $1,183.30   $1,183.40