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 $228.32   13.564%   $259.2893   $259.29   $259.30   $259.30  
  Purchase #2 $113.52   6.330%   $120.7058   $120.71   $120.70   $120.70  
  Purchase #3 $143.42   6.770%   $153.1295   $153.13   $153.15   $153.10  
  Purchase #4 $108.50   6.801%   $115.8791   $115.88   $115.90   $115.90  
  Purchase #5 $54.50   3.313%   $56.3056   $56.31   $56.30   $56.30  
  Purchase #6 $168.66   9.607%   $184.8632   $184.86   $184.85   $184.90  
  Purchase #7 $33.98   6.551%   $36.206   $36.21   $36.20   $36.20  
  7 purchase totals = $850.90       $926.3785   $926.39   $926.40   $926.40