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 $54.62   10.834%   $60.5375   $60.54   $60.55   $60.50  
  Purchase #2 $74.32   9.357%   $81.2741   $81.27   $81.25   $81.30  
  Purchase #3 $156.37   10.466%   $172.7357   $172.74   $172.75   $172.70  
  Purchase #4 $156.68   8.326%   $169.7252   $169.73   $169.75   $169.70  
  Purchase #5 $102.67   3.255%   $106.0119   $106.01   $106.00   $106.00  
  Purchase #6 $39.97   3.334%   $41.3026   $41.30   $41.30   $41.30  
  Purchase #7 $16.80   7.622%   $18.0805   $18.08   $18.10   $18.10  
  7 purchase totals = $601.43       $649.6675   $649.67   $649.70   $649.60