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 $23.87   11.767%   $26.6788   $26.68   $26.70   $26.70  
  Purchase #2 $19.26   9.308%   $21.0527   $21.05   $21.05   $21.10  
  Purchase #3 $217.36   11.319%   $241.963   $241.96   $241.95   $242.00  
  Purchase #4 $23.56   13.430%   $26.7241   $26.72   $26.70   $26.70  
  Purchase #5 $13.95   6.215%   $14.817   $14.82   $14.80   $14.80  
  Purchase #6 $245.98   7.666%   $264.8368   $264.84   $264.85   $264.80  
  Purchase #7 $212.73   13.867%   $242.2293   $242.23   $242.25   $242.20  
  7 purchase totals = $756.71       $838.3017   $838.30   $838.30   $838.30