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 $7.80   7.284%   $8.3682   $8.37   $8.35   $8.40  
  Purchase #2 $14.31   12.538%   $16.1042   $16.10   $16.10   $16.10  
  Purchase #3 $64.67   13.380%   $73.3228   $73.32   $73.30   $73.30  
  Purchase #4 $146.26   6.121%   $155.2126   $155.21   $155.20   $155.20  
  Purchase #5 $46.78   6.774%   $49.9489   $49.95   $49.95   $49.90  
  Purchase #6 $225.93   10.141%   $248.8416   $248.84   $248.85   $248.80  
  Purchase #7 $75.26   8.681%   $81.7933   $81.79   $81.80   $81.80  
  7 purchase totals = $581.01       $633.5916   $633.58   $633.55   $633.50