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 $242.45   3.474%   $250.8727   $250.87   $250.85   $250.90  
  Purchase #2 $88.55   3.726%   $91.8494   $91.85   $91.85   $91.80  
  Purchase #3 $116.57   9.435%   $127.5684   $127.57   $127.55   $127.60  
  Purchase #4 $206.94   13.600%   $235.0838   $235.08   $235.10   $235.10  
  Purchase #5 $169.82   4.839%   $178.0376   $178.04   $178.05   $178.00  
  Purchase #6 $109.31   4.273%   $113.9808   $113.98   $114.00   $114.00  
  Purchase #7 $57.37   4.681%   $60.0555   $60.06   $60.05   $60.10  
  7 purchase totals = $991.01       $1,057.4482   $1,057.45   $1,057.45   $1,057.50