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 $178.41   12.605%   $200.8986   $200.90   $200.90   $200.90  
  Purchase #2 $202.37   4.343%   $211.1589   $211.16   $211.15   $211.20  
  Purchase #3 $50.46   9.460%   $55.2335   $55.23   $55.25   $55.20  
  Purchase #4 $13.78   6.479%   $14.6728   $14.67   $14.65   $14.70  
  Purchase #5 $8.81   10.526%   $9.7373   $9.74   $9.75   $9.70  
  Purchase #6 $37.41   9.219%   $40.8588   $40.86   $40.85   $40.90  
  Purchase #7 $98.86   9.570%   $108.3209   $108.32   $108.30   $108.30  
  7 purchase totals = $590.10       $640.8808   $640.88   $640.85   $640.90