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 $11.92   13.277%   $13.5026   $13.50   $13.50   $13.50  
  Purchase #2 $118.28   7.251%   $126.8565   $126.86   $126.85   $126.90  
  Purchase #3 $111.69   8.843%   $121.5667   $121.57   $121.55   $121.60  
  Purchase #4 $192.93   6.138%   $204.772   $204.77   $204.75   $204.80  
  Purchase #5 $183.72   10.637%   $203.2623   $203.26   $203.25   $203.30  
  Purchase #6 $248.66   5.822%   $263.137   $263.14   $263.15   $263.10  
  Purchase #7 $50.73   5.749%   $53.6465   $53.65   $53.65   $53.60  
  7 purchase totals = $917.93       $986.7436   $986.75   $986.70   $986.80