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 $242.65   8.410%   $263.0569   $263.06   $263.05   $263.10  
  Purchase #2 $131.90   3.361%   $136.3332   $136.33   $136.35   $136.30  
  Purchase #3 $237.82   7.256%   $255.0762   $255.08   $255.10   $255.10  
  Purchase #4 $52.41   12.284%   $58.848   $58.85   $58.85   $58.80  
  Purchase #5 $107.47   12.292%   $120.6802   $120.68   $120.70   $120.70  
  Purchase #6 $80.48   11.768%   $89.9509   $89.95   $89.95   $90.00  
  Purchase #7 $45.40   12.218%   $50.947   $50.95   $50.95   $50.90  
  7 purchase totals = $898.13       $974.8924   $974.90   $974.95   $974.90