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 $6.63   4.288%   $6.9143   $6.91   $6.90   $6.90  
  Purchase #2 $24.99   10.388%   $27.586   $27.59   $27.60   $27.60  
  Purchase #3 $13.69   8.826%   $14.8983   $14.90   $14.90   $14.90  
  Purchase #4 $62.45   11.148%   $69.4119   $69.41   $69.40   $69.40  
  Purchase #5 $158.91   4.707%   $166.3899   $166.39   $166.40   $166.40  
  Purchase #6 $126.86   6.344%   $134.908   $134.91   $134.90   $134.90  
  Purchase #7 $95.33   13.272%   $107.9822   $107.98   $108.00   $108.00  
  7 purchase totals = $488.86       $528.0906   $528.09   $528.10   $528.10