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 $238.67   11.759%   $266.7352   $266.74   $266.75   $266.70  
  Purchase #2 $65.56   6.171%   $69.6057   $69.61   $69.60   $69.60  
  Purchase #3 $53.31   10.521%   $58.9187   $58.92   $58.90   $58.90  
  Purchase #4 $114.43   6.290%   $121.6276   $121.63   $121.65   $121.60  
  Purchase #5 $201.97   12.255%   $226.7214   $226.72   $226.70   $226.70  
  Purchase #6 $188.68   10.699%   $208.8669   $208.87   $208.85   $208.90  
  Purchase #7 $133.77   9.480%   $146.4514   $146.45   $146.45   $146.50  
  7 purchase totals = $996.39       $1,098.9269   $1,098.94   $1,098.90   $1,098.90