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 $78.65   13.818%   $89.5179   $89.52   $89.50   $89.50  
  Purchase #2 $115.68   5.236%   $121.737   $121.74   $121.75   $121.70  
  Purchase #3 $221.25   5.240%   $232.8435   $232.84   $232.85   $232.80  
  Purchase #4 $9.81   9.107%   $10.7034   $10.70   $10.70   $10.70  
  Purchase #5 $135.79   7.301%   $145.704   $145.70   $145.70   $145.70  
  Purchase #6 $167.57   5.699%   $177.1198   $177.12   $177.10   $177.10  
  Purchase #7 $56.83   10.765%   $62.9477   $62.95   $62.95   $62.90  
  7 purchase totals = $785.58       $840.5733   $840.57   $840.55   $840.40