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 $218.77   12.597%   $246.3285   $246.33   $246.35   $246.30  
  Purchase #2 $192.76   12.867%   $217.5624   $217.56   $217.55   $217.60  
  Purchase #3 $116.64   12.163%   $130.8269   $130.83   $130.85   $130.80  
  Purchase #4 $53.70   5.573%   $56.6927   $56.69   $56.70   $56.70  
  Purchase #5 $33.46   11.187%   $37.2032   $37.20   $37.20   $37.20  
  Purchase #6 $140.87   5.668%   $148.8545   $148.85   $148.85   $148.90  
  Purchase #7 $161.71   3.718%   $167.7224   $167.72   $167.70   $167.70  
  7 purchase totals = $917.91       $1,005.1906   $1,005.18   $1,005.20   $1,005.20