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 $17.79   4.276%   $18.5507   $18.55   $18.55   $18.60  
  Purchase #2 $225.45   5.733%   $238.375   $238.38   $238.40   $238.40  
  Purchase #3 $164.94   13.498%   $187.2036   $187.20   $187.20   $187.20  
  Purchase #4 $177.36   13.354%   $201.0447   $201.04   $201.05   $201.00  
  Purchase #5 $27.87   6.206%   $29.5996   $29.60   $29.60   $29.60  
  Purchase #6 $242.94   10.363%   $268.1159   $268.12   $268.10   $268.10  
  Purchase #7 $93.49   12.434%   $105.1145   $105.11   $105.10   $105.10  
  7 purchase totals = $949.84       $1,048.0040   $1,048.00   $1,048.00   $1,048.00