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 $90.27   10.692%   $99.9217   $99.92   $99.90   $99.90  
  Purchase #2 $88.50   13.470%   $100.421   $100.42   $100.40   $100.40  
  Purchase #3 $223.47   10.572%   $247.0952   $247.10   $247.10   $247.10  
  Purchase #4 $6.50   7.764%   $7.0047   $7.00   $7.00   $7.00  
  Purchase #5 $47.36   5.798%   $50.1059   $50.11   $50.10   $50.10  
  Purchase #6 $6.44   8.223%   $6.9696   $6.97   $6.95   $7.00  
  Purchase #7 $87.52   12.629%   $98.5729   $98.57   $98.55   $98.60  
  7 purchase totals = $550.06       $610.0910   $610.09   $610.00   $610.10