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 $29.37   6.871%   $31.388   $31.39   $31.40   $31.40  
  Purchase #2 $15.25   8.190%   $16.499   $16.50   $16.50   $16.50  
  Purchase #3 $189.94   4.808%   $199.0723   $199.07   $199.05   $199.10  
  Purchase #4 $100.71   7.657%   $108.4214   $108.42   $108.40   $108.40  
  Purchase #5 $227.86   13.437%   $258.4775   $258.48   $258.50   $258.50  
  Purchase #6 $8.30   9.347%   $9.0758   $9.08   $9.10   $9.10  
  Purchase #7 $34.96   10.166%   $38.514   $38.51   $38.50   $38.50  
  7 purchase totals = $606.39       $661.4480   $661.45   $661.45   $661.50