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 $100.30   6.109%   $106.4273   $106.43   $106.45   $106.40  
  Purchase #2 $74.40   12.397%   $83.6234   $83.62   $83.60   $83.60  
  Purchase #3 $168.46   3.786%   $174.8379   $174.84   $174.85   $174.80  
  Purchase #4 $145.81   12.570%   $164.1383   $164.14   $164.15   $164.10  
  Purchase #5 $93.52   6.661%   $99.7494   $99.75   $99.75   $99.70  
  Purchase #6 $243.30   9.404%   $266.1799   $266.18   $266.20   $266.20  
  Purchase #7 $94.46   13.673%   $107.3755   $107.38   $107.40   $107.40  
  7 purchase totals = $920.25       $1,002.3317   $1,002.34   $1,002.40   $1,002.20