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 $78.34   13.614%   $89.0052   $89.01   $89.00   $89.00  
  Purchase #2 $73.62   6.307%   $78.2632   $78.26   $78.25   $78.30  
  Purchase #3 $247.58   11.743%   $276.6533   $276.65   $276.65   $276.70  
  Purchase #4 $190.59   11.738%   $212.9615   $212.96   $212.95   $213.00  
  Purchase #5 $223.98   4.570%   $234.2159   $234.22   $234.20   $234.20  
  Purchase #6 $6.54   9.729%   $7.1763   $7.18   $7.20   $7.20  
  Purchase #7 $17.51   13.449%   $19.8649   $19.86   $19.85   $19.90  
  7 purchase totals = $838.16       $918.1403   $918.14   $918.10   $918.30