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 $232.79   12.362%   $261.5675   $261.57   $261.55   $261.60  
  Purchase #2 $174.98   10.799%   $193.8761   $193.88   $193.90   $193.90  
  Purchase #3 $44.96   6.780%   $48.0083   $48.01   $48.00   $48.00  
  Purchase #4 $59.44   5.278%   $62.5772   $62.58   $62.60   $62.60  
  Purchase #5 $206.97   5.656%   $218.6762   $218.68   $218.70   $218.70  
  Purchase #6 $68.42   5.458%   $72.1544   $72.15   $72.15   $72.20  
  Purchase #7 $127.60   11.230%   $141.9295   $141.93   $141.95   $141.90  
  7 purchase totals = $915.16       $998.7892   $998.80   $998.85   $998.90