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 $34.29   9.533%   $37.5589   $37.56   $37.55   $37.60  
  Purchase #2 $60.91   4.431%   $63.6089   $63.61   $63.60   $63.60  
  Purchase #3 $227.63   5.583%   $240.3386   $240.34   $240.35   $240.30  
  Purchase #4 $249.53   7.246%   $267.6109   $267.61   $267.60   $267.60  
  Purchase #5 $247.71   6.323%   $263.3727   $263.37   $263.35   $263.40  
  Purchase #6 $91.90   9.126%   $100.2868   $100.29   $100.30   $100.30  
  Purchase #7 $8.47   7.754%   $9.1268   $9.13   $9.15   $9.10  
  7 purchase totals = $920.44       $981.9036   $981.91   $981.90   $981.90