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 $56.80   11.835%   $63.5223   $63.52   $63.50   $63.50  
  Purchase #2 $232.25   7.268%   $249.1299   $249.13   $249.15   $249.10  
  Purchase #3 $206.89   8.121%   $223.6915   $223.69   $223.70   $223.70  
  Purchase #4 $30.82   11.363%   $34.3221   $34.32   $34.30   $34.30  
  Purchase #5 $165.33   9.548%   $181.1157   $181.12   $181.10   $181.10  
  Purchase #6 $187.29   3.516%   $193.8751   $193.88   $193.90   $193.90  
  Purchase #7 $10.94   9.780%   $12.0099   $12.01   $12.00   $12.00  
  7 purchase totals = $890.32       $957.6665   $957.67   $957.65   $957.60