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 $226.69   13.188%   $256.5859   $256.59   $256.60   $256.60  
  Purchase #2 $80.78   8.739%   $87.8394   $87.84   $87.85   $87.80  
  Purchase #3 $216.65   7.448%   $232.7861   $232.79   $232.80   $232.80  
  Purchase #4 $163.87   13.806%   $186.4939   $186.49   $186.50   $186.50  
  Purchase #5 $182.45   11.479%   $203.3934   $203.39   $203.40   $203.40  
  Purchase #6 $74.43   4.218%   $77.5695   $77.57   $77.55   $77.60  
  Purchase #7 $23.74   4.517%   $24.8123   $24.81   $24.80   $24.80  
  7 purchase totals = $968.61       $1,069.4805   $1,069.48   $1,069.50   $1,069.50