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 $206.36   12.733%   $232.6358   $232.64   $232.65   $232.60  
  Purchase #2 $127.48   9.574%   $139.6849   $139.68   $139.70   $139.70  
  Purchase #3 $211.37   5.667%   $223.3483   $223.35   $223.35   $223.30  
  Purchase #4 $236.72   6.648%   $252.4571   $252.46   $252.45   $252.50  
  Purchase #5 $182.38   10.653%   $201.8089   $201.81   $201.80   $201.80  
  Purchase #6 $191.65   6.581%   $204.2625   $204.26   $204.25   $204.30  
  Purchase #7 $24.51   4.471%   $25.6058   $25.61   $25.60   $25.60  
  7 purchase totals = $1,180.47       $1,279.8033   $1,279.81   $1,279.80   $1,279.80