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 $237.65   11.178%   $264.2145   $264.21   $264.20   $264.20  
  Purchase #2 $211.38   12.512%   $237.8279   $237.83   $237.85   $237.80  
  Purchase #3 $32.33   13.648%   $36.7424   $36.74   $36.75   $36.70  
  Purchase #4 $87.31   7.516%   $93.8722   $93.87   $93.85   $93.90  
  Purchase #5 $18.84   9.222%   $20.5774   $20.58   $20.60   $20.60  
  Purchase #6 $188.70   13.433%   $214.0481   $214.05   $214.05   $214.00  
  Purchase #7 $69.65   10.554%   $77.0009   $77.00   $77.00   $77.00  
  7 purchase totals = $845.86       $944.2834   $944.28   $944.30   $944.20