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 $120.30   7.783%   $129.6629   $129.66   $129.65   $129.70  
  Purchase #2 $47.34   8.319%   $51.2782   $51.28   $51.30   $51.30  
  Purchase #3 $16.79   6.373%   $17.86   $17.86   $17.85   $17.90  
  Purchase #4 $117.93   3.355%   $121.8866   $121.89   $121.90   $121.90  
  Purchase #5 $5.60   3.306%   $5.7851   $5.79   $5.80   $5.80  
  Purchase #6 $39.52   4.317%   $41.2261   $41.23   $41.25   $41.20  
  Purchase #7 $195.38   7.528%   $210.0882   $210.09   $210.10   $210.10  
  7 purchase totals = $542.86       $577.7871   $577.80   $577.85   $577.90