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 $29.62   6.773%   $31.6262   $31.63   $31.65   $31.60  
  Purchase #2 $36.35   6.378%   $38.6684   $38.67   $38.65   $38.70  
  Purchase #3 $133.51   9.300%   $145.9264   $145.93   $145.95   $145.90  
  Purchase #4 $73.59   10.675%   $81.4457   $81.45   $81.45   $81.40  
  Purchase #5 $94.88   12.155%   $106.4127   $106.41   $106.40   $106.40  
  Purchase #6 $78.78   10.518%   $87.0661   $87.07   $87.05   $87.10  
  Purchase #7 $89.62   8.418%   $97.1642   $97.16   $97.15   $97.20  
  7 purchase totals = $536.35       $588.3097   $588.32   $588.30   $588.30