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 $100.40   10.449%   $110.8908   $110.89   $110.90   $110.90  
  Purchase #2 $75.48   4.718%   $79.0411   $79.04   $79.05   $79.00  
  Purchase #3 $241.98   9.125%   $264.0607   $264.06   $264.05   $264.10  
  Purchase #4 $243.49   4.832%   $255.2554   $255.26   $255.25   $255.30  
  Purchase #5 $88.95   12.456%   $100.0296   $100.03   $100.05   $100.00  
  Purchase #6 $55.33   4.365%   $57.7452   $57.75   $57.75   $57.70  
  Purchase #7 $21.48   6.708%   $22.9209   $22.92   $22.90   $22.90  
  7 purchase totals = $827.11       $889.9437   $889.95   $889.95   $889.90