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 $249.99   5.804%   $264.4994   $264.50   $264.50   $264.50  
  Purchase #2 $183.88   11.453%   $204.9398   $204.94   $204.95   $204.90  
  Purchase #3 $51.79   10.247%   $57.0969   $57.10   $57.10   $57.10  
  Purchase #4 $179.28   11.344%   $199.6175   $199.62   $199.60   $199.60  
  Purchase #5 $120.92   4.809%   $126.735   $126.74   $126.75   $126.70  
  Purchase #6 $151.91   7.634%   $163.5068   $163.51   $163.50   $163.50  
  Purchase #7 $5.29   5.143%   $5.5621   $5.56   $5.55   $5.60  
  7 purchase totals = $943.06       $1,021.9575   $1,021.97   $1,021.95   $1,021.90