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 $244.54   10.724%   $270.7645   $270.76   $270.75   $270.80  
  Purchase #2 $60.60   8.141%   $65.5334   $65.53   $65.55   $65.50  
  Purchase #3 $171.42   9.587%   $187.854   $187.85   $187.85   $187.90  
  Purchase #4 $224.80   11.355%   $250.326   $250.33   $250.35   $250.30  
  Purchase #5 $56.55   8.489%   $61.3505   $61.35   $61.35   $61.40  
  Purchase #6 $164.78   11.381%   $183.5336   $183.53   $183.55   $183.50  
  Purchase #7 $167.54   10.629%   $185.3478   $185.35   $185.35   $185.30  
  7 purchase totals = $1,090.23       $1,204.7098   $1,204.70   $1,204.75   $1,204.70