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 $30.81   12.327%   $34.6079   $34.61   $34.60   $34.60  
  Purchase #2 $187.55   9.830%   $205.9862   $205.99   $206.00   $206.00  
  Purchase #3 $146.39   11.827%   $163.7035   $163.70   $163.70   $163.70  
  Purchase #4 $121.97   11.129%   $135.544   $135.54   $135.55   $135.50  
  Purchase #5 $59.30   12.657%   $66.8056   $66.81   $66.80   $66.80  
  Purchase #6 $241.98   13.297%   $274.1561   $274.16   $274.15   $274.20  
  Purchase #7 $125.78   9.733%   $138.0222   $138.02   $138.00   $138.00  
  7 purchase totals = $913.78       $1,018.8255   $1,018.83   $1,018.80   $1,018.80