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 $228.89   13.667%   $260.1724   $260.17   $260.15   $260.20  
  Purchase #2 $34.38   9.826%   $37.7582   $37.76   $37.75   $37.80  
  Purchase #3 $116.78   3.666%   $121.0612   $121.06   $121.05   $121.10  
  Purchase #4 $41.75   12.584%   $47.0038   $47.00   $47.00   $47.00  
  Purchase #5 $49.83   7.337%   $53.486   $53.49   $53.50   $53.50  
  Purchase #6 $232.29   4.409%   $242.5317   $242.53   $242.55   $242.50  
  Purchase #7 $211.57   3.535%   $219.049   $219.05   $219.05   $219.00  
  7 purchase totals = $915.49       $981.0623   $981.06   $981.05   $981.10