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 $250.74   3.687%   $259.9848   $259.98   $260.00   $260.00  
  Purchase #2 $121.43   4.272%   $126.6175   $126.62   $126.60   $126.60  
  Purchase #3 $63.28   4.432%   $66.0846   $66.08   $66.10   $66.10  
  Purchase #4 $206.62   11.273%   $229.9123   $229.91   $229.90   $229.90  
  Purchase #5 $179.87   3.156%   $185.5467   $185.55   $185.55   $185.50  
  Purchase #6 $29.80   12.861%   $33.6326   $33.63   $33.65   $33.60  
  Purchase #7 $241.55   9.725%   $265.0407   $265.04   $265.05   $265.00  
  7 purchase totals = $1,093.29       $1,166.8192   $1,166.81   $1,166.85   $1,166.70