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 $166.25   7.148%   $178.1336   $178.13   $178.15   $178.10  
  Purchase #2 $220.33   12.675%   $248.2568   $248.26   $248.25   $248.30  
  Purchase #3 $28.72   10.328%   $31.6862   $31.69   $31.70   $31.70  
  Purchase #4 $6.56   8.269%   $7.1024   $7.10   $7.10   $7.10  
  Purchase #5 $204.83   6.558%   $218.2628   $218.26   $218.25   $218.30  
  Purchase #6 $128.79   7.272%   $138.1556   $138.16   $138.15   $138.20  
  Purchase #7 $51.94   10.371%   $57.3267   $57.33   $57.35   $57.30  
  7 purchase totals = $807.42       $878.9241   $878.93   $878.95   $879.00