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 $130.58   7.307%   $140.1215   $140.12   $140.10   $140.10  
  Purchase #2 $144.86   3.509%   $149.9431   $149.94   $149.95   $149.90  
  Purchase #3 $149.64   3.628%   $155.0689   $155.07   $155.05   $155.10  
  Purchase #4 $106.69   12.495%   $120.0209   $120.02   $120.00   $120.00  
  Purchase #5 $138.90   3.499%   $143.7601   $143.76   $143.75   $143.80  
  Purchase #6 $114.42   5.812%   $121.0701   $121.07   $121.05   $121.10  
  Purchase #7 $46.45   7.732%   $50.0415   $50.04   $50.05   $50.00  
  7 purchase totals = $831.54       $880.0261   $880.02   $879.95   $880.00