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 $102.49   6.841%   $109.5013   $109.50   $109.50   $109.50  
  Purchase #2 $155.76   13.410%   $176.6474   $176.65   $176.65   $176.60  
  Purchase #3 $24.44   7.866%   $26.3625   $26.36   $26.35   $26.40  
  Purchase #4 $212.44   4.736%   $222.5012   $222.50   $222.50   $222.50  
  Purchase #5 $130.73   13.221%   $148.0138   $148.01   $148.00   $148.00  
  Purchase #6 $191.83   10.282%   $211.554   $211.55   $211.55   $211.60  
  Purchase #7 $22.73   12.345%   $25.536   $25.54   $25.55   $25.50  
  7 purchase totals = $840.42       $920.1162   $920.11   $920.10   $920.10