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 $229.57   12.463%   $258.1813   $258.18   $258.20   $258.20  
  Purchase #2 $196.77   13.496%   $223.3261   $223.33   $223.35   $223.30  
  Purchase #3 $59.51   8.387%   $64.5011   $64.50   $64.50   $64.50  
  Purchase #4 $241.69   13.870%   $275.2124   $275.21   $275.20   $275.20  
  Purchase #5 $147.52   13.203%   $166.9971   $167.00   $167.00   $167.00  
  Purchase #6 $238.99   8.113%   $258.3793   $258.38   $258.40   $258.40  
  Purchase #7 $44.72   11.524%   $49.8735   $49.87   $49.85   $49.90  
  7 purchase totals = $1,158.77       $1,296.4708   $1,296.47   $1,296.50   $1,296.50