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 $74.32   8.701%   $80.7866   $80.79   $80.80   $80.80  
  Purchase #2 $36.43   11.461%   $40.6052   $40.61   $40.60   $40.60  
  Purchase #3 $152.74   8.177%   $165.2295   $165.23   $165.25   $165.20  
  Purchase #4 $35.37   13.666%   $40.2037   $40.20   $40.20   $40.20  
  Purchase #5 $68.50   9.762%   $75.187   $75.19   $75.20   $75.20  
  Purchase #6 $16.91   10.869%   $18.7479   $18.75   $18.75   $18.70  
  Purchase #7 $197.74   13.499%   $224.4329   $224.43   $224.45   $224.40  
  7 purchase totals = $582.01       $645.1928   $645.20   $645.25   $645.10