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 $202.40   9.729%   $222.0915   $222.09   $222.10   $222.10  
  Purchase #2 $28.42   3.818%   $29.5051   $29.51   $29.50   $29.50  
  Purchase #3 $13.36   13.121%   $15.113   $15.11   $15.10   $15.10  
  Purchase #4 $171.56   12.273%   $192.6156   $192.62   $192.60   $192.60  
  Purchase #5 $153.79   12.501%   $173.0153   $173.02   $173.00   $173.00  
  Purchase #6 $27.56   13.496%   $31.2795   $31.28   $31.30   $31.30  
  Purchase #7 $199.76   8.692%   $217.1231   $217.12   $217.10   $217.10  
  7 purchase totals = $796.85       $880.7431   $880.75   $880.70   $880.70