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 $180.43   6.296%   $191.7899   $191.79   $191.80   $191.80  
  Purchase #2 $22.87   7.755%   $24.6436   $24.64   $24.65   $24.60  
  Purchase #3 $55.99   11.286%   $62.309   $62.31   $62.30   $62.30  
  Purchase #4 $170.42   13.862%   $194.0436   $194.04   $194.05   $194.00  
  Purchase #5 $110.67   3.818%   $114.8954   $114.90   $114.90   $114.90  
  Purchase #6 $201.57   13.527%   $228.8364   $228.84   $228.85   $228.80  
  Purchase #7 $177.76   3.302%   $183.6296   $183.63   $183.65   $183.60  
  7 purchase totals = $919.71       $1,000.1475   $1,000.15   $1,000.20   $1,000.00