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 $100.72   4.339%   $105.0902   $105.09   $105.10   $105.10  
  Purchase #2 $19.80   6.259%   $21.0393   $21.04   $21.05   $21.00  
  Purchase #3 $63.52   8.630%   $69.0018   $69.00   $69.00   $69.00  
  Purchase #4 $148.43   6.842%   $158.5856   $158.59   $158.60   $158.60  
  Purchase #5 $49.65   7.354%   $53.3013   $53.30   $53.30   $53.30  
  Purchase #6 $12.35   11.203%   $13.7336   $13.73   $13.75   $13.70  
  Purchase #7 $103.34   3.760%   $107.2256   $107.23   $107.25   $107.20  
  7 purchase totals = $497.81       $527.9774   $527.98   $528.05   $527.90