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 $109.45   11.336%   $121.8573   $121.86   $121.85   $121.90  
  Purchase #2 $77.45   8.542%   $84.0658   $84.07   $84.05   $84.10  
  Purchase #3 $99.43   7.382%   $106.7699   $106.77   $106.75   $106.80  
  Purchase #4 $61.85   6.475%   $65.8548   $65.85   $65.85   $65.90  
  Purchase #5 $22.62   10.784%   $25.0593   $25.06   $25.05   $25.10  
  Purchase #6 $22.84   10.788%   $25.304   $25.30   $25.30   $25.30  
  Purchase #7 $156.25   9.775%   $171.5234   $171.52   $171.50   $171.50  
  7 purchase totals = $549.89       $600.4345   $600.43   $600.35   $600.60