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 $203.71   3.424%   $210.685   $210.69   $210.70   $210.70  
  Purchase #2 $23.86   12.451%   $26.8308   $26.83   $26.85   $26.80  
  Purchase #3 $60.74   13.208%   $68.7625   $68.76   $68.75   $68.80  
  Purchase #4 $222.81   11.824%   $249.1551   $249.16   $249.15   $249.20  
  Purchase #5 $201.76   9.844%   $221.6213   $221.62   $221.60   $221.60  
  Purchase #6 $32.94   7.644%   $35.4579   $35.46   $35.45   $35.50  
  Purchase #7 $184.31   13.133%   $208.5154   $208.52   $208.50   $208.50  
  7 purchase totals = $930.13       $1,021.0280   $1,021.04   $1,021.00   $1,021.10