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 $225.66   13.683%   $256.5371   $256.54   $256.55   $256.50  
  Purchase #2 $166.76   9.153%   $182.0235   $182.02   $182.00   $182.00  
  Purchase #3 $169.57   13.808%   $192.9842   $192.98   $193.00   $193.00  
  Purchase #4 $129.76   11.501%   $144.6837   $144.68   $144.70   $144.70  
  Purchase #5 $213.34   3.111%   $219.977   $219.98   $220.00   $220.00  
  Purchase #6 $228.94   9.313%   $250.2612   $250.26   $250.25   $250.30  
  Purchase #7 $22.37   7.375%   $24.0198   $24.02   $24.00   $24.00  
  7 purchase totals = $1,156.40       $1,270.4865   $1,270.48   $1,270.50   $1,270.50