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 $235.39   10.182%   $259.3574   $259.36   $259.35   $259.40  
  Purchase #2 $131.92   9.849%   $144.9128   $144.91   $144.90   $144.90  
  Purchase #3 $77.77   13.717%   $88.4377   $88.44   $88.45   $88.40  
  Purchase #4 $66.74   13.608%   $75.822   $75.82   $75.80   $75.80  
  Purchase #5 $87.98   12.164%   $98.6819   $98.68   $98.70   $98.70  
  Purchase #6 $129.84   3.171%   $133.9572   $133.96   $133.95   $134.00  
  Purchase #7 $199.57   10.609%   $220.7424   $220.74   $220.75   $220.70  
  7 purchase totals = $929.21       $1,021.9114   $1,021.91   $1,021.90   $1,021.90