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 $28.61   8.131%   $30.9363   $30.94   $30.95   $30.90  
  Purchase #2 $126.52   9.796%   $138.9139   $138.91   $138.90   $138.90  
  Purchase #3 $68.72   3.229%   $70.939   $70.94   $70.95   $70.90  
  Purchase #4 $137.35   11.326%   $152.9063   $152.91   $152.90   $152.90  
  Purchase #5 $102.43   7.477%   $110.0887   $110.09   $110.10   $110.10  
  Purchase #6 $9.72   7.286%   $10.4282   $10.43   $10.45   $10.40  
  Purchase #7 $8.79   6.503%   $9.3616   $9.36   $9.35   $9.40  
  7 purchase totals = $482.14       $523.5740   $523.58   $523.60   $523.50