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 $231.93   6.123%   $246.1311   $246.13   $246.15   $246.10  
  Purchase #2 $121.35   6.855%   $129.6685   $129.67   $129.65   $129.70  
  Purchase #3 $167.92   5.198%   $176.6485   $176.65   $176.65   $176.60  
  Purchase #4 $60.38   10.865%   $66.9403   $66.94   $66.95   $66.90  
  Purchase #5 $233.33   7.408%   $250.6151   $250.62   $250.60   $250.60  
  Purchase #6 $112.99   6.373%   $120.1909   $120.19   $120.20   $120.20  
  Purchase #7 $220.48   12.533%   $248.1128   $248.11   $248.10   $248.10  
  7 purchase totals = $1,148.38       $1,238.3072   $1,238.31   $1,238.30   $1,238.20