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 $250.58   13.192%   $283.6365   $283.64   $283.65   $283.60  
  Purchase #2 $20.53   5.264%   $21.6107   $21.61   $21.60   $21.60  
  Purchase #3 $48.38   12.233%   $54.2983   $54.30   $54.30   $54.30  
  Purchase #4 $17.53   11.179%   $19.4897   $19.49   $19.50   $19.50  
  Purchase #5 $149.86   7.458%   $161.0366   $161.04   $161.05   $161.00  
  Purchase #6 $101.56   9.261%   $110.9655   $110.97   $110.95   $111.00  
  Purchase #7 $7.29   9.173%   $7.9587   $7.96   $7.95   $8.00  
  7 purchase totals = $595.73       $658.9960   $659.01   $659.00   $659.00