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 $244.46   10.558%   $270.2701   $270.27   $270.25   $270.30  
  Purchase #2 $143.80   13.254%   $162.8593   $162.86   $162.85   $162.90  
  Purchase #3 $61.60   13.479%   $69.9031   $69.90   $69.90   $69.90  
  Purchase #4 $69.61   8.511%   $75.5345   $75.53   $75.55   $75.50  
  Purchase #5 $210.57   7.657%   $226.6933   $226.69   $226.70   $226.70  
  Purchase #6 $211.72   3.295%   $218.6962   $218.70   $218.70   $218.70  
  Purchase #7 $232.82   9.745%   $255.5083   $255.51   $255.50   $255.50  
  7 purchase totals = $1,174.58       $1,279.4648   $1,279.46   $1,279.45   $1,279.50