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 $246.27   10.473%   $272.0619   $272.06   $272.05   $272.10  
  Purchase #2 $180.81   12.588%   $203.5704   $203.57   $203.55   $203.60  
  Purchase #3 $234.46   9.404%   $256.5086   $256.51   $256.50   $256.50  
  Purchase #4 $179.94   11.763%   $201.1063   $201.11   $201.10   $201.10  
  Purchase #5 $165.60   11.786%   $185.1176   $185.12   $185.10   $185.10  
  Purchase #6 $94.87   6.489%   $101.0261   $101.03   $101.05   $101.00  
  Purchase #7 $45.67   10.633%   $50.5261   $50.53   $50.55   $50.50  
  7 purchase totals = $1,147.62       $1,269.9170   $1,269.93   $1,269.90   $1,269.90