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 $38.74   4.405%   $40.4465   $40.45   $40.45   $40.40  
  Purchase #2 $49.92   7.645%   $53.7364   $53.74   $53.75   $53.70  
  Purchase #3 $171.66   3.693%   $177.9994   $178.00   $178.00   $178.00  
  Purchase #4 $106.28   7.737%   $114.5029   $114.50   $114.50   $114.50  
  Purchase #5 $150.92   12.855%   $170.3208   $170.32   $170.30   $170.30  
  Purchase #6 $127.93   3.841%   $132.8438   $132.84   $132.85   $132.80  
  Purchase #7 $172.25   9.417%   $188.4708   $188.47   $188.45   $188.50  
  7 purchase totals = $817.70       $878.3206   $878.32   $878.30   $878.20