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 $6.47   3.688%   $6.7086   $6.71   $6.70   $6.70  
  Purchase #2 $156.48   9.446%   $171.2611   $171.26   $171.25   $171.30  
  Purchase #3 $173.56   9.160%   $189.4581   $189.46   $189.45   $189.50  
  Purchase #4 $29.90   4.429%   $31.2243   $31.22   $31.20   $31.20  
  Purchase #5 $44.68   7.297%   $47.9403   $47.94   $47.95   $47.90  
  Purchase #6 $71.87   11.708%   $80.2845   $80.28   $80.30   $80.30  
  Purchase #7 $129.52   5.332%   $136.426   $136.43   $136.45   $136.40  
  7 purchase totals = $612.48       $663.3029   $663.30   $663.30   $663.30