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 $241.95   13.671%   $275.027   $275.03   $275.05   $275.00  
  Purchase #2 $97.76   4.850%   $102.5014   $102.50   $102.50   $102.50  
  Purchase #3 $25.59   10.310%   $28.2283   $28.23   $28.25   $28.20  
  Purchase #4 $34.90   4.814%   $36.5801   $36.58   $36.60   $36.60  
  Purchase #5 $54.53   6.806%   $58.2413   $58.24   $58.25   $58.20  
  Purchase #6 $246.51   9.541%   $270.0295   $270.03   $270.05   $270.00  
  Purchase #7 $83.33   5.412%   $87.8398   $87.84   $87.85   $87.80  
  7 purchase totals = $784.57       $858.4474   $858.45   $858.55   $858.30