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 $50.84   13.144%   $57.5224   $57.52   $57.50   $57.50  
  Purchase #2 $144.59   10.758%   $160.145   $160.15   $160.15   $160.10  
  Purchase #3 $84.74   8.207%   $91.6946   $91.69   $91.70   $91.70  
  Purchase #4 $23.59   7.466%   $25.3512   $25.35   $25.35   $25.40  
  Purchase #5 $18.25   8.142%   $19.7359   $19.74   $19.75   $19.70  
  Purchase #6 $148.67   10.224%   $163.87   $163.87   $163.85   $163.90  
  Purchase #7 $111.76   8.261%   $120.9925   $120.99   $121.00   $121.00  
  7 purchase totals = $582.44       $639.3116   $639.31   $639.30   $639.30