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 $9.99   9.334%   $10.9225   $10.92   $10.90   $10.90  
  Purchase #2 $11.43   6.769%   $12.2037   $12.20   $12.20   $12.20  
  Purchase #3 $177.52   12.189%   $199.1579   $199.16   $199.15   $199.20  
  Purchase #4 $207.82   11.802%   $232.3469   $232.35   $232.35   $232.30  
  Purchase #5 $169.26   10.276%   $186.6532   $186.65   $186.65   $186.70  
  Purchase #6 $80.77   11.428%   $90.0004   $90.00   $90.00   $90.00  
  Purchase #7 $91.49   3.322%   $94.5293   $94.53   $94.55   $94.50  
  7 purchase totals = $748.28       $825.8139   $825.81   $825.80   $825.80