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 $70.97   6.391%   $75.5057   $75.51   $75.50   $75.50  
  Purchase #2 $92.95   12.500%   $104.5688   $104.57   $104.55   $104.60  
  Purchase #3 $217.28   11.553%   $242.3824   $242.38   $242.40   $242.40  
  Purchase #4 $156.55   3.154%   $161.4876   $161.49   $161.50   $161.50  
  Purchase #5 $242.58   11.561%   $270.6247   $270.62   $270.60   $270.60  
  Purchase #6 $212.34   8.410%   $230.1978   $230.20   $230.20   $230.20  
  Purchase #7 $226.94   12.403%   $255.0874   $255.09   $255.10   $255.10  
  7 purchase totals = $1,219.61       $1,339.8544   $1,339.86   $1,339.85   $1,339.90