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 $53.35   3.614%   $55.2781   $55.28   $55.30   $55.30  
  Purchase #2 $175.37   10.647%   $194.0416   $194.04   $194.05   $194.00  
  Purchase #3 $198.39   4.347%   $207.014   $207.01   $207.00   $207.00  
  Purchase #4 $17.46   7.438%   $18.7587   $18.76   $18.75   $18.80  
  Purchase #5 $30.29   8.633%   $32.9049   $32.90   $32.90   $32.90  
  Purchase #6 $133.67   7.434%   $143.607   $143.61   $143.60   $143.60  
  Purchase #7 $171.40   5.607%   $181.0104   $181.01   $181.00   $181.00  
  7 purchase totals = $779.93       $832.6147   $832.61   $832.60   $832.60