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 $248.49   6.458%   $264.5375   $264.54   $264.55   $264.50  
  Purchase #2 $154.88   12.694%   $174.5405   $174.54   $174.55   $174.50  
  Purchase #3 $107.88   11.214%   $119.9777   $119.98   $120.00   $120.00  
  Purchase #4 $216.40   7.540%   $232.7166   $232.72   $232.70   $232.70  
  Purchase #5 $7.51   13.326%   $8.5108   $8.51   $8.50   $8.50  
  Purchase #6 $48.31   8.604%   $52.4666   $52.47   $52.45   $52.50  
  Purchase #7 $141.46   12.430%   $159.0435   $159.04   $159.05   $159.00  
  7 purchase totals = $924.93       $1,011.7932   $1,011.80   $1,011.80   $1,011.70