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 $212.47   12.451%   $238.9246   $238.92   $238.90   $238.90  
  Purchase #2 $45.60   10.474%   $50.3761   $50.38   $50.40   $50.40  
  Purchase #3 $26.26   8.780%   $28.5656   $28.57   $28.55   $28.60  
  Purchase #4 $122.49   8.182%   $132.5121   $132.51   $132.50   $132.50  
  Purchase #5 $143.42   8.790%   $156.0266   $156.03   $156.05   $156.00  
  Purchase #6 $111.56   13.747%   $126.8962   $126.90   $126.90   $126.90  
  Purchase #7 $235.49   13.859%   $268.1266   $268.13   $268.15   $268.10  
  7 purchase totals = $897.29       $1,001.4278   $1,001.44   $1,001.45   $1,001.40