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 $232.55   12.210%   $260.9444   $260.94   $260.95   $260.90  
  Purchase #2 $102.97   7.419%   $110.6093   $110.61   $110.60   $110.60  
  Purchase #3 $68.72   12.434%   $77.2646   $77.26   $77.25   $77.30  
  Purchase #4 $176.59   3.108%   $182.0784   $182.08   $182.10   $182.10  
  Purchase #5 $48.85   12.558%   $54.9846   $54.98   $55.00   $55.00  
  Purchase #6 $167.29   5.833%   $177.048   $177.05   $177.05   $177.00  
  Purchase #7 $201.74   12.167%   $226.2857   $226.29   $226.30   $226.30  
  7 purchase totals = $998.71       $1,089.2150   $1,089.21   $1,089.25   $1,089.20