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 $21.93   4.264%   $22.8651   $22.87   $22.85   $22.90  
  Purchase #2 $26.25   13.487%   $29.7903   $29.79   $29.80   $29.80  
  Purchase #3 $149.44   5.644%   $157.8744   $157.87   $157.85   $157.90  
  Purchase #4 $84.28   4.663%   $88.21   $88.21   $88.20   $88.20  
  Purchase #5 $125.30   5.368%   $132.0261   $132.03   $132.05   $132.00  
  Purchase #6 $28.49   6.407%   $30.3154   $30.32   $30.30   $30.30  
  Purchase #7 $168.44   9.573%   $184.5648   $184.56   $184.55   $184.60  
  7 purchase totals = $604.13       $645.6461   $645.65   $645.60   $645.70