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 $35.28   13.338%   $39.9856   $39.99   $40.00   $40.00  
  Purchase #2 $196.43   3.609%   $203.5192   $203.52   $203.50   $203.50  
  Purchase #3 $240.92   9.840%   $264.6265   $264.63   $264.65   $264.60  
  Purchase #4 $185.25   13.854%   $210.9145   $210.91   $210.90   $210.90  
  Purchase #5 $83.58   7.440%   $89.7984   $89.80   $89.80   $89.80  
  Purchase #6 $145.35   9.412%   $159.0303   $159.03   $159.05   $159.00  
  Purchase #7 $157.50   5.322%   $165.8822   $165.88   $165.90   $165.90  
  7 purchase totals = $1,044.31       $1,133.7567   $1,133.76   $1,133.80   $1,133.70