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 $100.70   7.222%   $107.9726   $107.97   $107.95   $108.00  
  Purchase #2 $145.71   10.124%   $160.4617   $160.46   $160.45   $160.50  
  Purchase #3 $209.60   10.360%   $231.3146   $231.31   $231.30   $231.30  
  Purchase #4 $236.54   10.108%   $260.4495   $260.45   $260.45   $260.40  
  Purchase #5 $233.26   8.513%   $253.1174   $253.12   $253.10   $253.10  
  Purchase #6 $129.42   6.809%   $138.2322   $138.23   $138.25   $138.20  
  Purchase #7 $133.32   4.673%   $139.55   $139.55   $139.55   $139.60  
  7 purchase totals = $1,188.55       $1,291.0980   $1,291.09   $1,291.05   $1,291.10