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 $19.54   3.394%   $20.2032   $20.20   $20.20   $20.20  
  Purchase #2 $112.37   5.122%   $118.1256   $118.13   $118.15   $118.10  
  Purchase #3 $35.79   5.758%   $37.8508   $37.85   $37.85   $37.90  
  Purchase #4 $74.68   9.867%   $82.0487   $82.05   $82.05   $82.00  
  Purchase #5 $11.50   12.689%   $12.9592   $12.96   $12.95   $13.00  
  Purchase #6 $25.94   5.483%   $27.3623   $27.36   $27.35   $27.40  
  Purchase #7 $189.70   8.104%   $205.0733   $205.07   $205.05   $205.10  
  7 purchase totals = $469.52       $503.6231   $503.62   $503.60   $503.70