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 $145.98   13.798%   $166.1223   $166.12   $166.10   $166.10  
  Purchase #2 $66.89   6.388%   $71.1629   $71.16   $71.15   $71.20  
  Purchase #3 $74.34   5.829%   $78.6733   $78.67   $78.65   $78.70  
  Purchase #4 $223.44   8.158%   $241.6682   $241.67   $241.65   $241.70  
  Purchase #5 $118.59   5.320%   $124.899   $124.90   $124.90   $124.90  
  Purchase #6 $207.48   11.438%   $231.2116   $231.21   $231.20   $231.20  
  Purchase #7 $16.71   6.524%   $17.8002   $17.80   $17.80   $17.80  
  7 purchase totals = $853.43       $931.5375   $931.53   $931.45   $931.60