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 $45.59   12.537%   $51.3056   $51.31   $51.30   $51.30  
  Purchase #2 $117.32   5.417%   $123.6752   $123.68   $123.70   $123.70  
  Purchase #3 $41.41   13.175%   $46.8658   $46.87   $46.85   $46.90  
  Purchase #4 $216.46   6.140%   $229.7506   $229.75   $229.75   $229.80  
  Purchase #5 $239.55   4.306%   $249.865   $249.87   $249.85   $249.90  
  Purchase #6 $107.29   3.102%   $110.6181   $110.62   $110.60   $110.60  
  Purchase #7 $112.33   5.389%   $118.3835   $118.38   $118.40   $118.40  
  7 purchase totals = $879.95       $930.4638   $930.48   $930.45   $930.60