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 $165.48   3.618%   $171.4671   $171.47   $171.45   $171.50  
  Purchase #2 $13.90   7.755%   $14.9779   $14.98   $15.00   $15.00  
  Purchase #3 $141.99   5.529%   $149.8406   $149.84   $149.85   $149.80  
  Purchase #4 $185.93   10.474%   $205.4043   $205.40   $205.40   $205.40  
  Purchase #5 $217.65   13.599%   $247.2482   $247.25   $247.25   $247.20  
  Purchase #6 $11.68   10.457%   $12.9014   $12.90   $12.90   $12.90  
  Purchase #7 $35.71   10.450%   $39.4417   $39.44   $39.45   $39.40  
  7 purchase totals = $772.34       $841.2812   $841.28   $841.30   $841.20