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 $7.50   9.348%   $8.2011   $8.20   $8.20   $8.20  
  Purchase #2 $50.35   10.139%   $55.455   $55.46   $55.45   $55.50  
  Purchase #3 $198.32   7.607%   $213.4062   $213.41   $213.40   $213.40  
  Purchase #4 $78.60   8.476%   $85.2621   $85.26   $85.25   $85.30  
  Purchase #5 $147.58   6.293%   $156.8672   $156.87   $156.85   $156.90  
  Purchase #6 $24.41   5.261%   $25.6942   $25.69   $25.70   $25.70  
  Purchase #7 $93.46   4.329%   $97.5059   $97.51   $97.50   $97.50  
  7 purchase totals = $600.22       $642.3917   $642.40   $642.35   $642.50