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 $249.86   6.760%   $266.7505   $266.75   $266.75   $266.80  
  Purchase #2 $20.60   7.561%   $22.1576   $22.16   $22.15   $22.20  
  Purchase #3 $41.81   11.844%   $46.762   $46.76   $46.75   $46.80  
  Purchase #4 $93.80   7.655%   $100.9804   $100.98   $101.00   $101.00  
  Purchase #5 $164.99   6.853%   $176.2968   $176.30   $176.30   $176.30  
  Purchase #6 $235.52   10.198%   $259.5383   $259.54   $259.55   $259.50  
  Purchase #7 $147.43   13.140%   $166.8023   $166.80   $166.80   $166.80  
  7 purchase totals = $954.01       $1,039.2879   $1,039.29   $1,039.30   $1,039.40