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 $120.64   13.249%   $136.6236   $136.62   $136.60   $136.60  
  Purchase #2 $32.28   4.232%   $33.6461   $33.65   $33.65   $33.60  
  Purchase #3 $243.73   10.217%   $268.6319   $268.63   $268.65   $268.60  
  Purchase #4 $68.44   12.441%   $76.9546   $76.95   $76.95   $77.00  
  Purchase #5 $103.58   8.408%   $112.289   $112.29   $112.30   $112.30  
  Purchase #6 $168.42   9.153%   $183.8355   $183.84   $183.85   $183.80  
  Purchase #7 $136.28   3.718%   $141.3469   $141.35   $141.35   $141.30  
  7 purchase totals = $873.37       $953.3276   $953.33   $953.35   $953.20