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 $116.29   4.136%   $121.0998   $121.10   $121.10   $121.10  
  Purchase #2 $57.26   11.504%   $63.8472   $63.85   $63.85   $63.80  
  Purchase #3 $116.38   3.290%   $120.2089   $120.21   $120.20   $120.20  
  Purchase #4 $175.66   10.539%   $194.1728   $194.17   $194.15   $194.20  
  Purchase #5 $146.67   7.298%   $157.374   $157.37   $157.35   $157.40  
  Purchase #6 $145.33   4.245%   $151.4993   $151.50   $151.50   $151.50  
  Purchase #7 $26.43   9.866%   $29.0376   $29.04   $29.05   $29.00  
  7 purchase totals = $784.02       $837.2396   $837.24   $837.20   $837.20