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 $44.64   12.310%   $50.1352   $50.14   $50.15   $50.10  
  Purchase #2 $162.33   12.744%   $183.0173   $183.02   $183.00   $183.00  
  Purchase #3 $117.76   6.866%   $125.8454   $125.85   $125.85   $125.80  
  Purchase #4 $183.42   10.640%   $202.9359   $202.94   $202.95   $202.90  
  Purchase #5 $176.75   11.593%   $197.2406   $197.24   $197.25   $197.20  
  Purchase #6 $117.47   12.246%   $131.8554   $131.86   $131.85   $131.90  
  Purchase #7 $25.89   7.558%   $27.8468   $27.85   $27.85   $27.80  
  7 purchase totals = $828.26       $918.8766   $918.90   $918.90   $918.70